I still don’t buy the idea that the antagonist can’t have an arc of their own
You keep saying I’m saying this but I’m not saying this. He can have an arc of his own. Vader has an arc of his own, but it’s primarily utility is as a supplement to Luke’s. Kylo has an arc of his own in both TFA and TLJ, but that doesn’t mean he’s a main character in either film (although you can argue he’s the co-lead of Last Jedi).
Well what I mean when I say “of his own” I mean that it doesn’t have to be in service of another character, which is what you seem to be suggesting it has to be.
What we’re disagreeing on is the idea that his arc could and should be a PRIMARY storytelling engine. Ultimately, as the villain, his arc needs to be in service to the main character’s (Rey). This happens in TFA and TLJ. It doesn’t really happen the way it needs to in TROS to realize the effect Abrams is hoping to achieve with his redemption.
I don’t see why it has to be the primary storytelling engine. Again, I don’t see how he can’t have an arc separate from the main character in the same way someone like Finn does, just because he’s the antagonist. Which is to say nothing of the fact that, like I’ve already said, there are plenty of ways they could have a Bendemption that has meaning for Rey’s character as well.