Everything was real except for the flash-sideways in this past season, which took place in a purgatory of sorts.
It was like the Dead Celebrities episode of South Park - they were all just waiting for each other to pass on in the real world so that they could move on to heaven or whatever all together... because their time on the island with each other was the most important time of their lives. It was also like Eternal Sunshine of the Spotless Mind's "meet me in Montauk" bit, except the memories are being triggered by other islanders (usually 815ers, since Jack and Juliet obviously had a life together but did not remember the island until they met Kate and Sawyer respectively).
The flash-sideways universe reminded me of the nexus from Star Trek Generations. Time runs differently there, so when Picard meets Kirk, who was sucked into it decades earlier, Kirk thinks he's just arrived. So the last scene in Lost is of Jack dying (therefore Kate, Sawyer, Miles, Richard, Claire, Lapidus, Hurley and Ben outlive him in the real world), even though he was the last to arrive at the church with the other islanders. We don't know when Hurley, Ben, and the Ajira plane passengers finally die, we just know that they will spend a lifetime in an idealized existence until they meet their constants from the 815 flight, at which point they'll head to that church to join up with the others. To each of them it'll seem like they'd all just arrived, even if in the real world, some people may have died decades later.
Jack may have been the last to get there because he took the longest to sort out his personal demons in that "purgatory" of idealized existence where he has a wife and son. He doesn't fully remember the island until he touches his father's coffin. Characters like Ben and Ana-Lucia are apparently not ready to leave this purgatory yet; Ben knows this about himself, while Desmond tells Hurley to carry on without AL. As a story-telling device, maybe it was done like this because they weren't the focus early on like the 815 fuselage survivors were. Maybe AL and Eko get their own church night.
I don't care for the exclusion of Walt and Michael, though. Desmond is a fail safe, so rules don't really apply to him, and that's why he was on 815 in the purgatory.
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