While I was playing through Bioshock Infinite, I was certain the whole affair was a dream inside Booker's head. It would have explained some odd things in the game that the parallel-universes thing didn't, like the mysterious corpse in the lighthouse, the near-fatal baptism, and the confusing shared history with Cornelius Slate. If anything, saying "it was all a dream" allows you to handwave any plot holes in the narrative, and Bioshock Infinite has quite a lot of them. I'm not saying the epilogue was terrible - there was a proper build-up to it - but I still think the dream idea would have felt a little better.