Here's what I say happened. The losties went back to 1977 to try to change what happened. In doing so they set off an atomic bomb and all died. Maybe that was there time to die. maybe not in 1977 but i mean at that point in their lives they were meant to die. But because they did manage to reset everything they will unknowingly have a second chance. But i fear that they will still all die when they reach that point in their lives (based on what Eloise explained to Desmond). but here's the catch, when they die in 1977, they died for they're own reasons. To help themselves really. But the next time around they will die for a more noble purpose, perhaps to save the island or even the world.