Frankly, the time of the old cast is over ...
One way to explain the disappearance of prime Spock from the Abrams-verse would be for him to return to the prime universe.
I hate to say it, but I don't think Mr. Nimoy is long for this world. Might be nice if he returned to the prime universe.
That stated, the more causal/historical connections there are between old and new Trek, the more the baggage of the old franchise will start piling at the feet of the writers.
What matters, however, is not the actor, but the character. There are a lot of people (most of them not yet born) who would keep the character alive and put their own twist on it.