I agree that I think originally the Island was meant to be purgatory. Given how it was going, I think the whole idea was that the characters were special to each other and the plane did take off and crash (but the island which nobody could reach or know about was purgatory). And all the flashbacks they had were them holding onto a past they couldnt let go of and the island and the people on their were there to help them move on,
The french lady being there for aaaaages and not being able to get help because nobody could really help her until she got over her own issues.
I do think the writers panicked because their endgame was guessed and they had to deny it. but they still incorporated the purgatory but had to do so in a sideways flash.
The series was fun, but once it was done, I didnt feel the satisfaction that I got after watching The Shield, The Wire or Breaking Bad. Im not comparing them, just that in a way, I felt Lost seemed like a waste and this is despite really enjoying the finale as an episode. It was a bizarre show but defo one that got lots of people talking.