As I said earlier, Spock "fixing" the timeline would essentially murder Kirk and replace him with the TOS version, alter the last 25 years of trillions of people and condemn Romulus and Remus to death in 2387.
Spock is not god. He doesn't have the right. STXIKirk has the same right to live as TOS Kirk had. The damage was already done by the time Spock arrived in the past.
All Spock could do was minimize further damage.
Spock is not changing the past as much as he is restoring it and the future.
"Restoring the future" at the cost of the lives of nuKirk and co.
Two wrongs don't make a right.
Who's to say the 2387 of the alternate universe will be any less prosperous than the other? Although there is no Vulcan, there is the chance to save the people of Romulus and Remus this time. You all saw how much history had diverged in 25 years. What would the next 129 hold?