I think, for better or worse, the ending of Lost is completely in line with the season and series that led up to it. Season six definitely got into a panic about landing the plane and shoved more explanations at me than I really needed (I think it's interesting that Lindeloff evolved through The Leftovers to "just let the mystery be"), but the show overall was preposterous, melodramatic, and pulpy. I just don't see how someone could hang through the 120 episodes before that and then find disappointment, especially if they were able to stick around from season 4 on.
It does also seem a lot of people conflate the church scene as meaning "the church is a purgatory they've all arrived in at once," and not "the church is the purgatory they will each eventually pass through;" I recently had a convo with a friend where they said the finale sucked because "it turned out they were all dead all along," which isn't what's trying to be conveyed there.
Also, the finale gets all the flak instead of the episode where Allison Janney "fills in" the backstory for the show's two meta antagonists by literally making up the rules of the world as she goes.