There seems to be alot of people divided on whether everyone survived the crash and the events of the 6 seasons were real, or whether everyone died in the crash and the island was their purgatory. There were many verbal hints in the alt timeline (Christans convo with Jack in the church, Kate saying shes missed Jack so much, Hurleys convo with Ben about being #1 and #2) which all suggested their time on the island was real, but alot of people don't seem satisfied because none of that is definitive and Jacks death scene was so similar to the first scene of the pilot.
I knew there was going to be some kind of additional visual proof to show whether Jack (and everyone else) died in or right after the plane crash or whether they survived for all that time on the island, went back to the real world, then back to the island. ITS THE SHOE. The shoe hanging from the bamboo right near where Jack dies, which turns out was Christians tennis shoe. Originally i thought it helped prove that Jack dies right when the crash happens in the jungle because the shoe is still there in the last episode, but it actually proves otherwise. When Jack originally wakes up in the pilot episode the shoe is hanging from the bamboo - a brand new white tennis shoe. When he wanders into the bamboo in 'the end' to die the same shoe is there old and worn like it had been there for years. BRILLIANT! I checked on several sites and they confirmed the same shoe hint. They survived.