It can be confusing that of all the characters in the film, it is Splinter who mentions Krang Two's lack of Character Development other than her desire to murder, until one realizes that Splinter himself was a Flat Character before he had his character development. At the beginning of the series the only characterization of Splinter was his laziness in a similar way to Krang Two whose only characterization is her desire to kill. In other words, the joke is taking on a completely new meaning since the character who did have a character development (Splinter) is getting angry at another who does not have any unique characterization (Krang Two).
While it's not shown much, Donatello and Michelangelo have more trust in Raphael as leader than Leonardo. Best exemplified in their argument scenes. After the failure of stopping the Foot Clan because of Leonardo flubbing the mission, Raph is going at it with Leo, Donnie and Mikey stood back, worried, but allowed Raph to make his point. When Leo was angry at Casey and blamed him for Raph being left behind with the Kraang, Donnie and Mikey hold Leo back from potentially hurting Casey. They trusted Raph well enough that he wasn't going to hurt Leo, since he had good reason, but they don't trust Leo enough that they wouldn't need to intervene, especially when they know Casey can't be blamed for it.
While it's implied that Raphael and Donatello's future counterparts died before the start of the movie because they don't appear in the opening scene and Casey mentions that all the turtles died, the fact that future Leonardo has Raphael's left arm hints strongly that at least the first one had a Cruel and Unusual Death or alternatively made a Heroic Sacrifice for Leonardo at some point. It is also seen that Casey never refers to Raphael as master like he does to the other turtles, implying that Raphael died before Casey was born or became a ninja.
Not Raph's arm- a mechanical prosthetic that simply resembles it. Listening carefully during the opening you can hear various mechanical sounds when Leo moves the arm. Though one might argue that perhaps it was originally a prosthetic made for Raph- as it seems too large and unwieldy to have originally been made for Leo.
Donnie and Raph are all but stated to be dead before the opening scene, and we can see April face down in the rubble during the Krang invasion. One person is completely absent from the bad future.What happened to Master Splinter?
Casey and the turtles didn't start changing the past — and thus the future — until after the key was stolen, meaning that in Casey's timeline, Leo's arrogance and showboating still led to the Foot claiming the key, except that time, there was no forewarning of what was coming. Everything that followed — the deaths of his family, the ruination of the world, all of it — was Future!Leo's fault, and he's been living with that guilt for over twenty years come the start of the movie.
The dimension where the Krang were trapped had a lot of destroyed exoskeletons but it has been established they are near indestructible so what could have done this? Another exoskeleton. That's right, the Krang were trapped in another dimension with no food or water, they probably had to resort to cannibalism to survive. Krang One's line about the others being too weak becomes harsher with this in mind.
It's also entirely possible that not every Krang is as evil as the three we see in the film proper, but any Krang that'd be reluctant to commit extreme violence were probably the first of their species to get picked off by their crueler neighbors. The three we see in the film are arguably the most ruthless of the Krang just by pure survival of the fittest.
Michelangelo's new powers are very impressive, but also very dangerous and could even end up hurting or taking the life of Michelangelo himself. If the boy doesn't receive proper training or Casey doesn't warn him how dangerous his ability to create portals can be, the question arises whether Michelangelo could end up self-harming or even killing himself at any second just by trying to open a portal at some point.
It's easy to overlook but when you think about it, Michelangelo could easily have died creating the portal to save Leonardo had it not been for the presence of his other older brothers. At the beginning of the film, an adult Michelangelo died in less than a minute by sacrificing all his vital energy to create a portal that would take Casey to the present and although he would lose his life in the process, at least he already knew what would happen to him by having further experience with his mystical powers. That alone is terrifying, but now he remembers that during the climax, a 13-year-old Michelangelo was opening a portal for the first time in his life without having any previous experience. Imagine if Raphael and Donatello had not been present at the time Michelangelo created the portal to help him, possibly the kid would have successfully made his goal but died at the exact moment of achieving it. Summarizing the above, the youngest turtle would have had a fairly quick death just like his future counterpart did, but unlike him, Michelangelo would die completely alone without even realizing it and without being able to see if he managed to save Leonardo (turning into ashes and without leave a trace of its existence).
Now see the previous point but from the point of view of the other turtles. Leonardo would have walked through the portal upon seeing him and happily reunited with his other brothers, but the three of them wouldn't know what happened to Michelangelo no matter when they looked for him. Leonardo telling the rest of the group about the portal that brought him back to earth and Michelangelo's disappearance, Casey would have put the pieces together and would know Michelangelo's fate because he himself had seen him die seconds before ending up in the present, having to tell his allies what really happened to Michelangelo and how he died. Think of Splinter's sadness and pain at losing his youngest son, April's confusion and denial at the thought of her close friend's death, Casey's guilt for telling Mikey about his portal powers, Raphael and Donatello's sadness and helplessness for not being there to save him, the horror and guilt Leonardo would have to live with because his actions indirectly took his little brother's life.
Krang One makes a comment about "the others" being weak while they were trapped in the Prison Dimension. The world was almost entirely destroyed by three Krang who, let's face it, were barely trying. The only time they suffered literally any kind of opposition was when the Turtles got their Mystic powers back and even then, Krang One wiped the floor with them without much effort after he stopped playing around. It's safe to say that the world would be doomed if the other Krang were here.
Leo was nearly trapped with Krang One in the Prison Dimension, and Krang One threatens to take out all of his frustration onto him, as Leo is the only one around to do so. Imagining a hypothetical scenario where Mikey didn't open a portal in time, and seeing just how downright cruel and vile Krang One is, there's no telling what manner of torture he could have put poor Leo through if he'd gotten the chance... and Leo would have been completely defenseless against it and unable to get any help. And he's only a teenager.
