I would think the same thing (that the island is purgatory) but the writers came right out and denied it from the very beginning. It was one of the first theories about the island ever since S1, actually. There are plenty of quotes from Lindelof and Abrams flat out denying it on several occasions over the years:
http://lostpedia.wikia.com/wiki/Purg...ebunked_theory)
http://www.losttvfans.com/page/Purgatory