and a super cosmic being ensures there's always a way out for them.
Unless an opposing super cosmic being who is about to lose everything else (in the context of this conflict) claims one of the Ellimist's heroes as a conciliatory prize. Crayak had tried this move before in MM3, making a deal that would result in a despised Animorph's death, but the Ellimist squirmed out of the corner and made sure there was a way out for Jake. Now, the stakes are much higher. The war is about to be decided, so the terms of the game have shifted. And Crayak says, "Okay, I'll concede the war, but I'm holding on to Rachel. She doesn't escape." This time Crayak makes sure there's no way out for the Ellimist.
At this point, was "The One" really necessary if you were ready to be done with it forever?
The One appeared right at the end to emphasize one of the major messages of the story: Your freedom and identity matter. Individuals matter. That's why K.A. made it such a big deal for the Animorphs to not morph sentient beings. That's why every book starts with "My name is..." That's why the Animorphs described the experience of morphing ants as being so horrifying, because ants have no identities. That's why the villains are known by numbers instead of names. That's why Jake always forced Ax and everyone else to vote on group decisions. That's why it was a big deal for the Animorphs to erase Visser Four's host in MM3. That's probably why we never learn the name of the Yeerk who steals Tom and becomes the primary villain at the end. That's why voluntary Controllers were described in book 1 as scum. That's why Rachel's last question was "Did I make a difference? Did my life matter?"
The whole series the Animorphs were fighting a force that was erasing people's individuality and identity, making them part of something greater, controlling their brains. That's why Marco describes The One as representing every evil, every corruption, and as causing people's brains to shut down. The One absorbs individuals, erases any control or significance they have, and makes them part of a greater being. The One itself has no name, it is simply The One.
Ending the series with the Animorphs encountering The One, who tells them to surrender their selves, and deciding to "Ram the Blade ship" reminds readers at the end of the story what the story was about. It's also completely badass, and the Animorphs (and Rachel in particular) had always been totally badass.