Ultimately, Luke in the OT was an aspirational character because he struggled. He repeatedly stumbled and fell over the course of the trilogy. But while he questioned himself, he never slinked away from his sense of purpose, nor did he waver from his drive to help others.

The issue is that TLJ tries to show a Luke who has stumbled and ends the film with him “getting back on his feet” and being a hero again, but going into a suicidal exile for 6 years and convincing yourself that the Jedi deserve to die out is not some momentary weakness. That’s a massive character shift that contradicts two of Luke’s defining traits in the OT: his compassion, and his unwillingness to give up. Luke could have been shown as depressed or struggling while still keeping those core traits intact.

Letting the Jedi end because he doesn’t want more Sith to emerge is sort of understandable, but when there are Dark Siders actively terrorizing the galaxy, the least he could do is try to correct the situation before ending the Jedi for good. And Luke could of course renounce the Jedi while still assisting against the First Order in other ways.