Well this thread is certainly interesting now!
I find it really interesting how the writers did succeed in avoiding the "emotionally unsatisfying" options of negating either of the realities, or having the flashsideways characters be mere dopplegangers of the real characters.
That left us with only the future of the Losties, but in an unreal place where the world's amnesia about the famous 815 survivors could be explained. I think I would have preferred it to be a hallucination/test vs the afterlife.
Something about the flash-sideways determines which of the candidates takes Jacob's place.
And that would have solved the problem that people are bitching about - the flashsideways had no purpose in the story.
Lindelof established in a podcast that the date of flight 815 in the flashsideways is the same as the date in the pilot episode, September something 2004.
Tsk tsk. He lied.