The filmmakers could’ve given Luke a different motivation for staying on Ahch-To, while not having him be out-of-character, contradictory to his already-established character traits (like redeeming other family members) or undoing his arc from the original trilogy so they can re-do that arc all over again. I mean, why would he be looking for the first Jedi temple, as mentioned by Han in The Force Awakens?
the fact that people think “redeeming family members” is a character trait and not an action of pure good shows that it needed to be told this way xP
it’s not redo-ing a character arc if he learns and develops further than the original lesson
The arc may have a different resolution, but it’s still mostly the same arc. Luke almost kills a family member, but realizes that he shouldn’t kill him. The scenes have different outcomes, but Luke has the same motivation. Luke renounced the darkness within him at the end of ROTJ, it doesn’t make sense for him to repeat the mistake he made with Vader. And I don’t think it “needed to be told this way,” there are a thousand other possible reasons for Luke’s exile that could have actually expanded his character instead of feeling like a regression.