My problem with the idea of either Kylo or Vader surviving their respective redemptions is that even in a fantasy film universe as wide open as the one Star Wars occurs in - I simply don’t see a future for either character where they’re not immediately murdered as retribution for their crimes, either by the respective governments, or by mob rule. Asking audiences to accept that family members (or possible lovers) can forgive the terrible in those they hold dear is one thing. Asking the rest of that fictional universe to get in line behind them is… a lot. And really, really pushing the suspension of disbelief.
Their only real future along those lines is self-exile. Which was actually pitched in the case of Kylo, but ignored by Abrams as a possibility. Kylo communing with the Force for the rest of his life, in solitude, on Ahch-To makes sense to me. Kylo wandering the galaxy like some sort of do-gooder Ronin? I don’t see it. It’d just be wave after wave of people trying to take him out because of, you know, the whole genocidal dictator thing.
I could see self-exiling either character for literally DECADES, and then being summoned out of that exile as a last ditch “you’re our only hope” sort of hail mary for whoever the heroes of that follow-up story were. i.e. “I know of one man who could help. Maybe. But you’re not gonna like it”
That’s one hell of a long game to play. But it’s probably the only real shot at a plausible “redemption/atonement” storyline for those characters. You need a ton of time and a whole lot of distance from the events of the movie they redeemed themselves in. Give them a chance to become legends/spectres, and then they finally get a chance to do the right thing for the right reaons on a large scale.
But in the case of Kylo… I just don’t see him surviving like 50,000 assassination attempts if he lived past The Rise of Skywalker. He’d have to hide out in a place where no people go for a very, very long time.