The End End Theory
Did the Island exist or were the Losties dead all along (since the crash)? I say NO. Proof: Desmond, Penny and Juliet were in the church and Ben was outside at the end. Unless they were also in the 815 plane crash then Jack (and/or the rest) would not have known them. Therefore the Island was ‘real’ and so were the adventures.
Granted there were many little holes, unresolved issues and mysteries in the storyline which could render this logic inapplicable. But these characters were major and if you accept the Island was Purgatory, then the architectural foundation of the story is compromised. Not to mention this theory was denied by the writers from my readings.
Secondly, it appears the flight did make it off the island in which the people onboard (Kate, James etc) may have lived for some time afterward. Proof: Kate says to Jack “I missed you for so long”. Did she die of old age? Possibly, remember Christian told Jack the Island adventure was the most significant event in all their lives. Hurley and Ben’s discussion about their being a great #1 and #2 respectively, inferring they lived for some time after Jack died as protectors of the Island.
The only other explanation (also dismissed by the writers): All events occurred in the theatre of Jack’s mind as he was dying. I think from the reviews, most believe the sideways world was more likely being played out in Jack final moment of life if anything. This would parallel the sideways world being Purgatory as opposed to the island. Somehow I prefer to think it was the cast’s waiting area before moving on to the afterlife or new jobs. Why some people were not in the church? Perhaps they were not dead yet or already moved on (eg Frank, Miles etc).
Speaking of new jobs, who knows Jack, may not even be dead. Maybe he became the new Smoke Monster (I stole this from another contributor), which would have given Hurley and Ben something to do right away in a spinoff series serving as the plot for the 1st episode.
Sorry of these ideas have already been theorized. If so, now I’m like you!