I wouldn't be at all surprised if Abrams' master plan is to do three movies on the assumption that his universe re-merges with the original by the time he's done and well before the 24th C, with no one the wiser that the past was "wrong." Except for Uhura and the Spocks of course, and everyone they tell.
Mr. Laser Beam wrote:
^ The Mirror Universe is different. It's not an alternate timeline created by time travel; it always existed. It's a completely separate universe. The same people's births and lives are simultaneously no more, and no less, likely to happen in the MU than they are in any other.
The Abrams U could be analogous to the MU, a parallel universe that always existed. If there are two, then why stop there? why not three? Why not three quadrillion? it's easier to envision the Abrams U as being one of many parallel universes that naturally exist than start worrying about butterfly effects.
But dramatic logic overrides everything, which is why I think the two universes really are being treated as different from the MU, and that they will merge together before Abrams is done.