You guys familiar with Schrodinger's cat? Basically, a cat is placed in a box with a device that has a 50% chance of poisoning and killing the cat. Until we open the box, the cat is both alive and dead.
The key part of the experiment is that the box is observation proof and blocks all forms of radiation. So it is literally unknowable whether the cat is alive or dead without opening the box and so, both outcomes are true until the box opens.
Conventional time travel in Trek involves altering a single timeline by moving through time within the universe.
That is to say, Kirk and crew aboard the HMS Bounty are going backwards in time. Key word there: IN time.
So while they're traveling between the 23rd century and 1985, they're going backwards IN time. At any point inbetween, with sophisticated enough observation methods, you could observe the HMS Bounty on its course backwards through time. (Technically, this means that for the bulk of the movie, they're in two places at once. In San Francisco and also aboard the HMS Bounty heading back towards the point they arrived at.)
So while Zefram Cochrane is testing his warp drive, the HMS Bounty is also present in the Sol system, difficult to track, but on its way back towards 1985.
Now... with Spock and Nero, I propose that they actually travelled back OUT OF time. They were outside the universe.Inobservable. Both alive and dead for all intents and purposes to everyone inside the universe.
In one eventuality, they're alive and arrive in the 23rd century, where they mess with history.In another eventuality, they're dead and the universe goes on without them. Both are true because they were in a Schrodinger's cat-like state inside the event horizon of the black hole when the Red Matter Capacitor went off.
However, if any third party could look into that black hole, they would be definitive alive or dead and one of the two universes would assert itself and the other would be wiped out. Meanwhile, if contact were made between the two universes, they would both cancel eachother out, like Schrodinger's cat observing its own corpse. (Technically, the live cat is the only one that would cancel itself out.)
So running with this, the reason they created an alternate timeline is because there's no way an outside party can confirm whether they survived entering the black hole. If they survived, the JJ-verse sequence of events happens. If they didn't survive, the Prime timeline continues on. Both can be simultaneously true so long as no one attempts to verify which one is true.