The writers wrote whatever they wanted to and in the end basically said "everyone dies anyway sooner or later so what happened on the island doesn't matter, but look! Everyone is happy! And Jack is dying but not alone, Vincent is there! Its sad but happy!".
That is a really cheap way to get away with that they established. Many Lost fans have your line of thinking as well leading me to believe they know they got suckered into watching a show in which the chief gimmick was that eventually everything will be explained but never really does. But its an emotional happy ending, so they take that and run with it and come up with a bunch of fan theories to cope with in the meantime to find answers.
Essentially that means anyone will defend and keep watching an improv show that has mythical creatures, alchemy, zillions of alternate universes, and people coming back from the dead infinite times so as long as it has convinsing drama and a happy ending. And if seems confusing to explain, that's purely the writer's fault. And I doubt it would've been hard to explain the smoke monster, the importance of aaron and walt, the Numbers, and so on. I could come up with some explainations on spot.
I wouldn't have minded if they ended it with just one new mysery but jesus, to disregard everything else?