The character dynamics of THAT movie are drastically different (and kind of exciting) more-so than the sort of “we know he’s going to turn good by the end here” vibe that TROS and DOTF both had. The idea that the rest of the trio HAVE to interact with him now would be really, really interesting to see play out.
This is a big reason why I like this idea! A movie where Kylo/Ben is having to interact with Rey, Poe, Finn, etc. is entirely different from their interactions in the last two films. In TROS, the interactions between Rey and Kylo don’t feel any different from the last two films. It definitely would have provided plenty of material for interesting scenes that could deal with forgiveness and atonement in a way that Vader’s redemption never did, setting TROS more apart form ROTJ.
Like you said, I feel like most people went into this film expecting Kylo to be redeemed, so why not do it early on, play around with character interactions we would never get otherwise, and then maybe Rey actually turning to the dark side and then Ben saving her at the end be the big “hurray” moment. After their fight, Ben could have been the one to get the pep talk from Luke, and they could’ve made amends. I’m not trying to take away from Rey’s story, but I think Rey and Ben could’ve been dual-protagonists whose stories reflected each other, then finally coalesce in the final act. Rey’s story could’ve been similar to Anakin’s, who slowly is seduced with the promise of saving the one’s he cares about from dying. The weight of being the galaxy’s savior, of being the last Jedi, could be weighing heavily on her throughout the film, which leads her down the dark path starting around the midpoint or end of act 2, reflecting Ben’s turn at the end of Act 1.