Star’s decision to destroy the magic and its consequences
Consequences:
There is a question of whether the setting is full of beings dependent on the realm of magic to live and whether Star's destruction of it resulted in a mass death of such beings.
It is important to note that the realm of magic is not responsible for all of the supernatural or “magical” things the characters encounter. Beings with powers that are inherent to their species explicitly exist. Examples include Ponyhead’s ability to levitate and Tom’s pyromancy. The show also includes different universes with fantastical properties, such as the strange time flow in the Neverzone or the solid clouds on which the Ponyhead kingdom is built. Both of the Ponyhead related examples are shown to continue after the destruction of the realm of magic, confirming their status as self-contained supernatural phenomena. Thus, it cannot be assumed that any fantastical elements are connected to the realm of magic unless they are specified as such.
The next thing to note is that the mewmans did not gain access to the magic on their own. They were given the wand by Glossaryck who then had to teach them how to use it. In fact, all uses of magic that demonstrably come from the realm of magic (wand spells, the high commission and other constructs, the dimensional scissors) can be traced back to Glossaryck. This doesn’t rule out the possibility that some unrelated people could gain access, but there are several of factors that make it unlikely.
First is the observable properties of the magic itself. Its golden liquid form seems to only enter a given universe at a single point and is not shown to have any physical effect on the world around it. A person encountering it would have no reason to assume that it can be used for magic. The background magic field aspect also doesn’t have any clear means of presenting itself as magic. It seems to be entirely undetectable and supernatural sensitivity doesn’t appear to exist. The characters only learn of the Toffee’s corruption as a result of active spells beginning to fail. On top of this, overexposure to the magic is shown to cause amnesia and/or insanity. This would severely inhibit a person’s ability to discover magic by experimentation.
The second reason is related to the dimensional scissors. Unlike other magic, plenty of people have scissors, including monsters. Monsters who would have a vested interest in traveling to other dimensions and gaining access to the magic for themselves. Yet, the Mewmans have been in power for about four hundred years without this happening. This seems extremely unlikely if other means of gaining access were at all feasible.
The final argument against Star’s actions causing mass death is Toffee’s corruption of the magic. By the time Star rebooted the realm of magic, there was only a tiny spec of uncorrupted magic left. All of the constructs and spells that Glossaryck and the Butterflies had made were gone by that point. Any unrelated beings dependent on magic to live would likewise have been killed. As such, any potential genocide would have happened at this point and not when Star destroyed the magic later. The blame would be on Toffee and possibly Glossaryck (there is evidence that he meant Toffee to be the one to destroy it in the first place). Star’s rebooting of the magic did not bring them back to life. Everything that exists in the realm of magic afterward was created new by Star at this point. The magic high commission were deliberately revived afterwards by pouring liquid magic back into their bodies. The only things that came back automatically were the wand spells. This can be attributed to the fact that the wand works like a computer and stores every spell it casts in its memory. Plus, all of these are externally created constructs rather than independent beings (who would not have an obvious means of revival).
Ultimately, there is no compelling evidence that the consequences of the realm of magic’s destruction by Star are anything other than what is depicted in the show: the destruction of constructs made by Glossaryck and the Butterflies and the removal of a means interdimensional travel.
Reasoning:
Star’s belief that it is too dangerous for anyone to be trusted with is not merely based on the current situation, but a long-term pattern of abuse. Beyond creating the Solarian Warriors, the queens of Mewni have destroyed at least one universe, created a spell that disintegrates anything it touches, and frozen time for the entire multiverse. Anyone with access to this magic is a multiversial threat, not just a local one. It’s tempting to write the above acts off as poor decisions made by bad individuals, but to do so is to ignore the effect that power has on people. A person with access to that magic (even by proxy) is capable of getting anything they want, without effort, for their entire lives. It is inevitable that a person with such power will come to feel entitled to reward without effort and become callous towards those less fortunate than themselves (a cursory glance at real-life people with extreme wealth or status supports this). This is demonstrated in-universe by the mewmans who participate in Mina’s scheme. A proper peace between them and the monsters necessitates cooperation and hard work towards a compromise. As such, Star organizes diplomatic events to aid this process. The mewmans overwhelmingly refuse to engage and instead seek out Moon and pester her to use magic to make the problem go away. Eventually, Moon too succumbs to the prospect of using magic for an easy out.
Given the level of destruction it enables and the attitude that possessing such power creates in people, Star is within reason to point to the magic itself as a problem. In a situation with no clean solutions, Star makes the choice that saves the most provably existing lives in the moment while ensuring that such a situation (or worse) could never happen again.
