Perhaps, to show why Spock was necessary, I should phrase this as a Q and A.
Q: How do you start over with Star Trek?
A: You go back to the most identifiable elements of Star Trek, The Enterprise, her crew (Kirk/Spock/McCoy/Etc.) and start over with them afresh.
Q: How do you do this, while preserving the 40+ years of stories, history and lore that have been established on screen?
A: You find a link to the rest of the franchise, and make that link an emotional and logical cause for the changes you have to make.
Q: What character/event/whatever would fit with established canon, and have the emotional and logical resonance to do this?
A: Spock is the only character that the general public generally link to Star Trek, and who's ultimate fate is unestablished, with the emotional weight to pull it off.
See... you're really just bullshitting yourself here. If it had been just a solid, well made with lots of explosions 'generic' sci-fi flick it would've still done solid business. People like sophomoric humor (see Police Academy) and big 'splosions (any buddy cop movie) which this movie had in spades.
So you didn't need Leonard Nimoy... you didn't need William Shatner... you didn't need Patrick Stewart, you needed empty calories to satisfy the audiences 'hunger'. That is what this Star Trek film got right... nothing more.
And if they had left Nimoy in the retirement home, they wouldn't have needed to twist the film's story logic like a pretzel.
Take the Humor out of your thinking here. Nimoy's Spock's presense is immaterial to this, regardless of your opinions either way.
The film's logic was actually far more straightforward to me and most viewers than your opinion implies, and if you believe that Nimoy was not needed to pass the tource, or make the film legitimate, then I welcome your good, serious solution, irrespective of the elements of the movie you did not like.
PROVE ME WRONG.