i think the writers want us to get the idea that they all died at one point in there lives whether it was on the island for example charlie in season 3, Juliet in season 5 or jack in 6, the very last episode, or way after they left in the airplane (kate, sawyer, richard, ben, miles, frank) then they all created the ‘sideways world’ because they weren’t ready to move on without each other. i’m certain the crash actually happened. Hope this is the answer you wanted!