Magnifico's modus operandi of only granting wishes that benefit himself feels poetic when one takes into account how it contrasts with the iconic wishing star. If one recalls Jiminy Cricket's song about said-star, "When you wish upon a star, makes no difference who you are." This makes a nice Foil to Magnifico, who only grants other people's wishes conditionally.
Magnifico's decision to grant a wish of a woman who wants to make the best clothes in Rosas fits his doctrine of only granting wishes that are useful for him: One more business is created for him to tax.
It would also benefit him personally because a tailor who makes the best clothes would logically work making the best clothes for the most important and powerful person in the country, that being Magnifico himself.
Magnifico initially attempts to justify his decision not to grant certain wishes by claiming that the consequences could be "unpredictable", such as regarding the wish of Asha's grandfather Sabino that he be able to inspire others as dangerous because Magnifico couldn't be sure that Sabino wouldn't inspire others to, for example, rise up against Magnifico himself. This ultimately reflects Magnifico's own arrogance and self-centeredness, implying that he fears people would rise up against him because that's essentially what he would do with such power in their position.
In Magnifico's Villain Song, the lyric "I let you live here for free and I don't even charge you rent!" may sound like a Department of Redundancy Department sentence, but it actually highlights Magnifico's hypocrisy of his benevolence. Previously, he claims that he's good by giving the clothes of Benito to a person who may or may not need them and then volunteering Henry to save a house on fire rather than himself. And his solution for people who lost their homes was just to let them live in the kingdom for free but he never stated anything about actually repairing their houses or fixing their troubles. He bloats his heroics with redundant descriptions that sound natural and helpful but are actually ridiculous once broken down, which is just like how he treats their wishes given to him.
Despite King Magnifico having other apprentices in the past, only Asha seems to care that there are wishes that will never be granted. However, then you remember that all his previous apprentices had their wishes and the wishes of their families granted as a perk of their employment. Meaning, unlike Asha, they were willing to swallow their pride and then promptly retired once they got their wishes, also keeping quiet about all the ungranted wishes so as to not get on Magnifico's bad side.
People wish on stars because they think those stars have the power to grant them. The "We are stars" line plays into defying this idea—if the wisher is already a star, then they already have the power to grant their own wishes.
The plot is set into motion when Asha realizes that her grandfather is among the many people who will never have their wish granted by Magnifico, which leads to her summoning Star and eventually standing up to the King and his corruption. Even without it being granted by magic, Sabino's wish to inspire the next generation came true when he inspired his granddaughter.
Magnifico's entire system of taking wishes from people, holding them for years, and then occasionally granting one in a massive public spectacle. He could easily just spend a few hours a day granting wishes. In fact, that would be way more efficient if he actually cared about the wishes of the people. Instead, he set up a system that lets him grant an occasional wish and be publicly praised for it while hoarding the rest.
The Star method of granting people wishes (Star does help you, but you still have to put the work into it) is this when you rewatch the older films. Even in films like Pinocchio or Cinderella, the protagonists are just given the tools needed to help make their wish a reality, but they still need to put in the work (with some help).
If you compare Star's design with that of Disney's previous incarnation of the Wishing Star, the Blue Fairy, then you realize that the Blue Fairy and Star are, respectively, basically Disney's version of Rosalina and Luma!
Magnifico's reasoning for not granting Sabino's wish is that his desire to be "inspirational" could inspire the people to - among other things - start an uprising against their King. While it's meant to show Magnifico's self-centered and far-fetched logic when granting or withholding the wishes, given the many inherent flaws in the system and how he uses it to hoard his power, it's very likely that granting Sabino's wish would have inspired the people to finally realize those flaws, and fight to change them as Asha soon does. Magnifico wouldn't have hesitated to grant such a simple and benevolent wish if he truly was a good ruler who had nothing to hide, but as he is not, he was completely correct in his assessment that such a wish would potentially be dangerous to himself.
This is even more apparent when you look deeper into the wish since it's actually one that cannot be granted: not only would be sketchy as best to force inspiration on other people, but if the wish granted the inspiration it would be the magic to be inspirational, not Sabino. Not only does this reinforce how he doesn't consider the wishes by themselves, but only how they relate to him, it also indicates how he doesn't see the issue in using magic to get people's approval.
Magnifico refused to grant Sabino's wish on the grounds that he could use his music to inspire people to rise up against him. Due to events resulting from that, that ends up happening, as after he reveals his evil nature and goes off the deep end, Asha performed two songs to inspire people to rise up against him (her friends in "Knowing What I Know Now" and the whole kingdom in "This Wish (Reprise)").
Magnifico being fearful of Sabino's wish makes a lot more sense when one considers when the movie takes place, I.E: the 1200's. This is because Sabino was specifically shown wanting to inspire people through music, and occupations such as musicians and playwriters were much more politically relevant back then. Since most people were illiterate during the 1200's, songs and plays were the main source of not just entertainment but also news, as well as being a common source of propaganda and as a result could be used to inspire outrage and insurrection with ease, and since the masses had no means of fact checking back then and could only take people at their word, that meant that those people could use their songs and plays as a means of spreading slander and falsehoods to their heart's content to instigate a revolution and there'd be nothing to prevent people from believing them, and since Magnifico has no knowledge as to the kind of person Sabino is, he has no reason to doubt that Sabino would do exactly that if he got his wish granted.
There's actually a brilliance to having Dahlia to be the first one to answer Asha's call to action in the final battle. Dahlia is The Smart Guy; she operates on firm logic and what's possible...so when she steps up and stands against Magnifico, her friends realize that while they may not have an easy chance, they do have one because Dahlia would only stand up if it wasn't a hopeless endeavor. Having Queen Amaya herself not only defend Dahlia against Magnifico's attack but also stand with her only boosts this knowledge.
Magnifico is probably not just one of the most narcissistic of Disney Villains, but he's likely the most sociopathic as well. Since he chooses which wish to grant and which to decline, it's possible that some of those wishes he's rejected from his subjects include asking for their loved ones to be healed of a terminal illness, only for him to reject the wish because it did not benefit him. As a result, he probably has an outrageously high body count simply because he can't be bothered.
On that note, the only details we get regarding the death of Asha's father is that he "got sick" and passed away several years ago, and we don't know specifically when Asha's mother made her wish, nor do we ever learn what she wished for. It could very well be that she wished for her husband to get better, or even to see him again after his death, but Magnifico either rejected the wish outright or he held off on it until it was too late, only to then literally crush her wish completely to spite Asha.
A Recipe for Adventure reveals that her wish is to see Asha shine (which she does), but who's to say that a person can't have multiple wishes? If so, it's highly possible that Magnifico crushed both just to get back at Asha for defying him.
It's explicitly stated and plot-relevant that a person only has one wish as their driving desire in life, so Sakina's wish for her daughter is her only one.
Who's to say that Asha isn't the first person to have figured out the truth about Magnifico's wish-granting system? Moreover, who's to say that Magnifico hasn't killed any of his potential apprentices to prevent them from exposing him to the kingdom?
Magnifico being imprisoned in the mirror has a brief flash of the Magic Mirror from Snow White... meaning he becomes that Mirror. Since he and Amaya have no children, presumably a relative takes the throne when she dies leading to the Good King... who then marries the Evil Queen as his second wife. Yes, that's right - Amaya and Asha cause the events of Snow White to occur with their actions.
Alternatively, Amaya will become the Evil Queen of this world (that character is sometimes referred to in comics as Grimhilde but it is not her official name). She already has access to all of Magnifico's equipment and books, including the forbidden one, and no one challenges her keeping the throne in the first place despite her being complicit in his crimes prior to his Jumping Off the Slippery Slope; who's to say she won't become cold and vain the way he did and from there just as ruthless, while keeping up a kindly mask to woo a widowed Good King? Though it is nerfed in that she TRIED to find a way to save Magnifico until it was clear it was impossible.
The book A Recipe For Adventure mentions that before he gave his wish away, Simon was incredibly active, helped his friends, and trained to achieve his dream of being a knight—but now all he does is sleep and worry a piece of him is missing. Even without his wish being crushed like what happened to others, simply having his wish taken is close to depressive symptoms.
Valentino wishes for "a world where all mammals wear clothes and are equal". While it may at first not sound so distressing but more shocking given how Zootopia is revealed to be connected to the rest of Disney movies, the real kicker comes when you read through his sentence again, and see that he only said mammals. Does this mean that in the world of Zootopia, only mammals are sapient despite the movie's creators alluding to the existence of bird and reptile cities?
Amaya mentions that she was fooled by the love she felt for Magnifico and watched the good in him melt, hinting that on some level she had noticed his darker tendencies and let them slide before. It also paints troubling questions about the nature of their marriage, as Magnifico makes everything about himself even before tapping into the forbidden magic and clearly expects Amaya to passively agree with him.
When did the person who wanted "a nanny for [their] horrible children" make their wish? It can't be when they were 18 unless Rosas has no issue with child marriage, so there's a very good chance it was quite some time ago (when the person moved to Rosas) and the children have already grown up, making its return useless. And how many other wishes are now useless due to the passage of time, or even death? A lot of wishes will end up not making their makers happy upon their return, or might even still be floating about without owners — and the latter could be used by evildoers now that people know what happens when they're crushed!
A Recipe for Adventure mentions Asha and Dahlia met 6 years before the events of the film. Asha's father had been dead for 5 years before the events of the film, meaning their first year of friendship was tested by the loss of a parent—and possibly two, depending on when Dahlia's grandmother died.
Sabino is 100 when the film begins, meaning he probably doesn't have long left to live by the time it's over.
The main issue of the plot is that, despite the name and what the characters say, Wish isn't actually about wishes, it's about goals, since that's what Magnifico actually works on (as the opening narration states, he extracts the desire that drives you), which is why they are all things the characters can achieve on their own merits. The most apparent case is the Expanded UniverseNovelizationA Recipe for Adventure revealing that Asha's mother's wish was for everyone to see Asha glow (something that she can influence by raising and guiding her) rather than for her husband to recover from illness (something outside her power). However, the Wishing Star (and Fairy Godmother, since that's what Asha becomes) operate on actual wishes, things that are completely outside of one's control and as such need to be granted by other forces/people: Cinderella cannot overcome class differences and an abusive family locking her out of resources through effort or hard work, same for Pinocchio becoming actual flesh and blood. It happens in the movie itself when the star gives Valentino a voice and sapience. By conflating the two concepts both the plot and message end up being undercut, and it even damages the goal of the movie as a celebration of Disney, since it shows that Disney either doesn't understand its own ideas or doesn't care about them.
Part of the problem might be that in recent decades, Disney films (and stories like "Cinderella" more generally) have often been accused of encouraging passivity in their viewers (particularly young girls) by suggesting they cannot change their own fates themselves and Hard Work Hardly Works, and the filmmakers wanted to avoid a story in which that would pretty much be the message, so they made the "wishes" more-or-less easily attainable things. But again, this was something that should have been ironed out at the screenplay stage. Ironically, one of the film's promotional partners was the Make-a-Wish Foundation, which does understand the concept of wishes quite well, being a charitable foundation that arranges for terminally ill children to do something that would have been impossible with just their families' means (such as visiting Disney Theme Parks or meeting celebrities).
Another issue is how the people of Rosas come off in their choice to give their goals up to a lottery system. If they honestly believe they can't learn to be seamstresses, musicians, captains, etc. that suggests a lack of intelligence (especially combined with nobody, not even the supposedly "sharp-witted" Asha, seeming to figure out that not all wishes will eventually be granted) unless they have a disability of some kind working against them, given Rosas is a prosperous kingdom where want is unknown. If the characters are not dimwitted, this means they are willingly giving up a vital part of themselves because they want/expect Magnifico to do the hard work for them. In any case, the audience is expected to sympathize with a bunch of lazy townspeople and be upset with the hard-working Magnifico for being cautious about risking everything he's created and gained.
If Magnifico supposedly knew from the start just how "dangerous" using the forbidden book of dark magic was, why did he even have it in his castle in the first place, let alone in such an easily accessible (for him) location where he could easily be tempted to use it!?
It's presented as a Godzilla Threshold; something to use if the alternative is the kingdom's inevitable doom. He probably had noble intentions when he first obtained it, but years of power and adoration twisted his mindset such that he's more eager to resort to it at the first threat to his reign.
