So yeah, I'm probably totally nuts:
I totally believe that this movie takes place in Tony Manero's head on his deathbed. You bookend the movie with "This Is The End" for crying out loud. I know somebody in the live show remarked that the whole movie had a dream-like quality to it. That's why no matter what he does, he comes off ahead of where he was before. Perhaps he's in a coma, alone with whatever might be running through his head as his life is slipping away from him.
When he disappears at the end of the movie, that's him dying in real life. He's "Staying Alive" just long enough to live out his dream in his head, and then he struts into the afterlife.
Ok, now that I've typed it out, it seems really stupid, but when I was listening to the show in afternoon traffic, it made sense.