In DTI: Watching The Clock, Commander Juel Duncan traveled back to December 2381, to stop the assassination of Nan Bacco. He even doubled back on her timeline to have the double Bacco help him stop it. What was such a big deal in the following couple of years that Duncan had to come back and fix it if she was meant to die so soon afterward?
That's not how it works. If history records things happening a certain way, then that's the way they're supposed to happen, as far as the Temporal Integrity Commission is concerned.
Besides, nobody exists in isolation. We interact with other people every day, and our presence affects their actions, which in turn affect the actions of others, etc. It's the domino effect. And someone with the power of the Federation President is going to make decisions that set a lot of dominoes in motion. Changing the length of that president's term in office, even by a couple of years, could change a great deal of the ensuing history, even if the key changes are quite a few dominoes down the line.
Bacco has made some important decisions in the interim, though. For instance, I doubt relations with the Romulans would be as good now if she and Praetor Kamemor hadn't pursued their talks. Who knows whether a different president would've been willing to extend that trust? Alternatively, a different president might've responded differently to the Breen's slipstream espionage attempt, and by now either the Breen would have slipstream or the Federation and the Pact would be at war.