I think that maybe the flash sideways was limbo/ purgotory, but not exactly how we usually think of them. I think that their limbo/ sideways world is where they learn that they need each other. The sideways world maybe allowed time for all of them to die and then move on together. I just came up with this so tell me if someone disagrees or wants to add to what i'm saying. I love how Darlton didn't answer all of our questions. If they had, we would be disappointed and wouldn't we sharing our ideas and thoughts about what really went on.