First, no they did not die in the plane crash. You can cue that into the statement by Hurley at the end, something to Ben of you were a great number two, etc. They lived, the island WAS just what Jacob said it was, it mattered. It was the ‘plug’ in ‘hell’ and if the light were removed, the ‘plug’ would come unstuck and the real world would cease to exist. However, they each had such a profound effect on each other on the island, they waited for each other in their ‘flash sideways’ world, which was just an imagination of reality until they moved on. Kate, Sawyer, and the rest on the plane did get off the island and lead their new lives. Sure there are questions… like Aaron… never ‘born’. Walt, who is so special, and has no father or mother now. The list could go on. But, I think it somewhat makes more sense now.