It does seem like there was an issue where they felt they needed a primary, ultimate evil, antagonist, because they wanted to redeem Kylo and they felt that wouldn’t be possible if Kylo was that ultimate baddie.
In the OT you had the ultimate evil with the Emperor to contrast to Vader, and another example with Avatar: The Last Airbender, Ozai was the ultimate evil to contrast with Zuko. Both Vader and Zuko found redemption while the ultimate evil was defeated. It almost seems like a narrative rule. But I think it would’ve been compelling to keep Kylo as the ultimate evil, and ask the question, “Can the ultimate evil be saved?”
It’s a lack of imagination. Especially if you’re going to have Kylo be the one to redeem himself, of his own volition (which is what basically happened in this film anyway), you don’t need a bigger bad to face off against. He should be able to look in the mirror and realize for himself what he’s doing is wrong, not just what someone else is doing is wrong.