Quite simple really; 29th century Starfleet, as shown in the VOY episode Relativity, would send some temporal agents to 2387 (like it was there job) or when Nero arrived in the 23rd century, and do some temporal higgedgy piggedy or hocus pocus and mend the timeline.
So if Nero was somehow stopped from going back in time, then Vulcan would never have been destroyed, and the timeline (or quasality) of the prime universe is thus preserved. I'm certain Starfleet of the 29th century would abide by the Temporal Prime Directive and mend the timeline.
The point is this can allow for both universes to exist, since JJ Abram's universe exists so as long as the temporal discontinuity continues (which in essence means this universe can go on forever more as the flow of time would be different), and the Prime universe will exist because 29th century Starfleet fixed the timeline.
So we don't have to worry about the timeline being altered; both universes apply!
So if Nero was somehow stopped from going back in time, then Vulcan would never have been destroyed, and the timeline (or quasality) of the prime universe is thus preserved. I'm certain Starfleet of the 29th century would abide by the Temporal Prime Directive and mend the timeline.
The point is this can allow for both universes to exist, since JJ Abram's universe exists so as long as the temporal discontinuity continues (which in essence means this universe can go on forever more as the flow of time would be different), and the Prime universe will exist because 29th century Starfleet fixed the timeline.
So we don't have to worry about the timeline being altered; both universes apply!