If I can be honest, I think that the ultimate and most important reason for Kylo dying is this: Rey isn’t a Skywalker, only has that tenuous pseudo-romantic tie to Kylo, and would easily be seen as a demoted by a surging Ben Solo because he’d be “The Skywalker of the ST” without any dispute.
Vader was, on at least some kind of level, more sympathetic than Kylo, more popular, and contributed genuine dramatic power to Like’s story over his own. But Vader still had to die. Kylo/Ben would be a liability towards Rey going forward, and her story doesn’t benefit from a living Ben.
Ben honestly would have a had a better shot at living with Rey Solo/Skywalker, because then the audience could at least have a selfish reason to care about Ben fr Rey’s sake that more people would buy into.
Right now, the only selfish reason to buy into Kylo for Rey’s sake is of you endorse the Reylo idea... and let’s be honest, that was a niche fandom, and niche hatedom, and a total-non factor that probably risks being more hated as time goes on, not loved.
