Forgive my novice knowledge of how temporal mechanics works, but if Nero didn't create a new timeline replacing the original timeline, then why, in the Prime Universe, would there be a Department of Temporal Investigations that made sure that nothing was changed? They're all parallel universes, after all, right?
No, I reject that theory. You can't create a timeline and kill people essential to that timeline and expect that timeline to go on existing unchanged. Vulcan was destroyed as a result of Nero's incursion. Where does that leave Tuvok or any of the other younger future Vulcans? Were they still born? Do they still exist? Spock was immune, for what I imagine would be much the same way the Enterprise E crew still existed after the Borg tried to assimilate the past, protected by the Temporal Wake. But all of his compatriots from the future. Gone. Or changed. Will Kirk still get sucked into the Nexus and ultimately killed? Does it matter if there's no Guinan? No Enterprise D? How can they exist when the past is changed? What would have happened if Nero went to 1945? Would you and I still be here? Think about it.