Does anyone else get the feeling that in the episode Flashes Before Your Eyes the flash he had is the same thing that the FST is? Not the same world, but the same concept. I think Desmond died when he turned the key and went to his own kind of Purgatory.
Purgatory in Lost seems to revolve around a certain moment in one's life that ends up differently. For the FST it's the Oceanic 815 flight. In Flashes it's Desmond breaking up with Penny. In both "realities" Eloise seems to have inside knowledge and is actively working to convince Desmond of something. In Flashes she's trying to get him to repeat the OT events and break up with Penny, which he ultimately does. This somehow reincarnates him, though he doesn't know quite what happened. This is important to Eloise here because Desmond has work to do to get the end goal she wants. In the FST, Eloise tries to prevent Desmond from going down the path of discovery. She finally has what she wants and doesn't want to ruin it.
Perhaps the bomb blast affected the Source and that somehow made it so in their Purgatory they cannot crash on the Island. That way things go on as they normally would as if the plane never crashed, just as Desmond's would have if he stayed with Penny. But they all did things that made them remember the OT, just as Desmond kept getting hints that he did it all before. He bought the ring when he wasn't supposed to, which kept the Purgatory flash going instead of bringing him back. Then like a day or so later he threw the ring in the river and that triggered the end of the flash.
I've been of the mind that they actually died and are going to the afterlife for real now, but perhaps they will be reincarnated after all? I didn't like the theories of reincarnation that I've heard, but could it be the kind of reincarnation that Desmond went through? Maybe Jack will wake back up in the bamboo thicket and the rest of them will be there, no matter what they did in life after The End.
I'm not sure if I like this part of the theory still, but there seem to be parallels with Flashes Before Your Eyes. The failsafe blast triggered Desmond originally, and the electromagnetic blast that Widmore did to him also seemed to get him to sense the FST in the same way he saw the Flashes reality. Maybe him waking up there is his reincarnation moment.
I believe I remember a podcast sometime this season where they talked about Flashes and its relevance to the FST. "Flashes before your eyes" is a reference to death, and the Purgatory we see is like living your life up to a certain point over again, but you get to change things. Dunno. Discuss.