Who says purgatory isn't real? I would not call either the island experience or the Sideline "not real" or a dream. I would regard both as real events and the real interactions of the characters, and in the case of the Sideline, a more real story than we thought was originally being told - that of an alternate universe where the plane never crashed. THAT would have been a "fake" story. Just because something happens to someone after they die does not make it any less real. The afterlife shown in this episode is not one I believe in, but I don't believe in an island in the South Pacific populated by polar bears, a smoke monster and a community of fertility researchers & runway builders, either. I believe the Sideline as an afterlife is every bit as real and part of the "lives" of the characters as the other things we see happen on the island and off.

Regarding my comments about the insignificance of the island or its actually being purgatory, for all intents and purposes, the characters mostly went there to die. When the focus of your life becomes making the preparations for your death, for all intents and purposes your life is over. There is no greater significance or importance to anything the characters did on that island (aside from the six who flew off) beyond making the personal connections to sustain the afterlife of the Sideline, and give them some common ground in death. It's basically the story of a retirement home (or death row) with more special effects and action sequences than you might ordinarily expect, and more of a happy ending than could reasonably be expected of any long-term story of such a facility.