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).

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.

This isn’t an OT vs ST thing, it’s a basic storytelling structure thing. It’s not specific to Star Wars. The antagonist CAN become the protagonist, but there’s a lot of work that needs to be done to get that antagonist to that point, and it’s got to be done in a way that informs and supports THE MAIN CHARACTER primarily before that transformation can be successfully completed. Otherwise you’re trying to switch horses midstream and it’s almost impossible for that not to play as a disservice to the main character, and to the detriment of your overall story.