Not a bad theory at all. I have heard that theory before and it works for me....
The film ends up being one of two things: Either A) an alternate timeline, in which case everything prior to the Narada exiting the black hole is supposed to be 100% identical to the TOS we know and love (with the exception of CGI/make-up upgrades) but it isn't, its been changed significantly; or B) A reboot, in which case things are changed for the better, such as the size of ships, their crew compliments, and their technological abilities, but yet the Spock we know and love somehow comes back in time anyway.
I would have been happy with an alternate timeline. I would have been happy with a reboot. The problem is that we got NEITHER.
I look at it this way: this is an alternate timeline of an alternate timeline. In timeline one, we get a progression -- more or less -- from "The Cage" in TOS through "All Good Things" in TNG and then Generations. Then, in First Contact, 1701-E goes back in time chasing the Borg and allows Zephram Cochrane and Lily Sloane to see advanced starship designs. Sloane crawled around in one and saw model representations of others. These clues probably gave Cochrane enough information to make small improvements to his original concepts. This then leads to an altered timeline that at least goes through Star Trek: Enterprise, which featured a larger, possibly more advanced starship than may have existed at that time originally. Archer's mission might never have been, and the whole Temporal Cold War may have been started by a misunderstanding of the events in First Contact.
This new timeline would have continued to develop in the 23rd century in a manner probably very much like that seen in TOS, although the ships would continue to be larger and more advanced. NX-01, after refit to NX-01.5 with a secondary hull and other refinements, might have delayed the production of Constitution-class starships. But ultimately, the Enterprise might still have been crewed by Kirk, Spock, Scotty, etc.
But twenty years before that, Nero and the Narada came from the future and nailed the Kelvin, creating the second divergence from the original timeline. Where then does Spock-Prime come from? It's tempting to say he comes from the original timeline, but he recognized a man as Montgomery Scott who clearly looked nothing like Scotty. So I'm happy to shrug my shoulders and go along with the idea that Spock-Prime actually hails from Archer's future, not the future we've already seen.
Another pet theory of mine is that Chekov in this alternate timeline isn't the same Chekov from the original show. In the original show, Chekov was much younger than his crewmates, but in Trek '09, the difference didn't look as great. I suspect his parents got together a bit earlier and Pavel was born sooner. I actually prefer Trek '09's Chekov. With all due respect to Mr. Koenig, Yeltzin's Chekov seems so much more Russian.
Another theory I have heard which I also like is that this movie, from start to finish, takes place in a universe independent of the prime universe and not one that has branched off from the prime universe. So in that universe Nero did change the time line but it was the time line of that universe and not the prime universe. This also means both Nero and Old Spock originated from the future of that specific universe and they were not from the prime universe.
So in other words Trek 09 is completely set in a universe that is not connected to the prime universe in any way.