I usually get these wrong, but here goes.
I used to think that the Flash Sideways were the destination point for the series. That in the original timeline Desmond would do something that would shift the space/time continuum to create the timeline where the crash didn't happen, i.e. the Flash Sideways timeline.
But that would destroy all the character development of the past six years, making it meaningless.
Now what I think is that the Flash Sideways are the original timeline and that Sideways Desmond is going to work his magic mojo to rearrange the space/time continuum to bring about the timeline that we all know and love--the one where the crash happened.
So in other words in the crash timeline, what happened happened. Those who have died, really did die.
