Just a theory. Let's say that everyone aboard Ocieanic 815 all died and the whole series they were in a different stage of afterlife kind of like the "flashsidways". Perhaps they keep jumping from one afterlife to the next and at the end of the series was there final jump to Heaven or what ever you want to call it. It makes sense because how did a lot of the passengers survive without a scratch on them. Plus they had some weird happenings that wouldnt happen in real life, smoke monsters, island disappering, time travel etc. Perhaps all the other passengers that appeared to have died on the plane, their spirits traveled to a different afterlife. This keeps reapiting until each spirit finds what they are looking for and can move on.