This shouldn't be so hard to grasp. Everyone's favorite character from any series other than TOS flat out will not exist in this new timeline. The further out from the first temporal incursion, the more likely it is that something will disrupt the conception of each and every beloved TNG, DS9 and VOY character, until it becomes statistically impossible for any of them to exist. None of these people will be around to avert the dire threats to the Federation they did in the original timeline.
Spock won't be warning the Feds about stardate who gives a shit when some guy named Q throws the Enterprise D and Captain Picard into the path of the Borg. He just tells them that the Borg, as a species, exist, where they're most likely at and about any tech/tactics his timeline developed to deal with them. End of story.
Does he warn them about Kirk dying on the Enterprise B? Why they fuck would he? That's such a specific event, dependent upon way too many variables to possibly occur the same way again, even if there were an Ent B and Kirk was aboard during a similar disaster. The existence of advanced civilizations and their inclinations "Borg are out there and they're fucking evil" and natural hazards "Collapse the Bajoran wormhole NOW" and general ideas "Genesis doesn't work" are much more likely to remain true regardless of what ripples out from the divergence.
And it doesn't matter when or even if a single one of these numerous dire threats show up, only that they have a protocol in place that if threat A appears or you run into species B, solution X worked once, so why not try it again before we go it on our own just for the fucking sake of going it on our own?
If I got thrown back in time to 1979 resulting in some equivalent-crazy shit happening, causing a divergent timeline, you think it would be better if I didn't warn the US government about helping Al Qaeda get started in Afghanistan or backing Saddam against Iran, or buying into Kim Jong Il's bullshit again and again or any number of other shitty things that're going to wind up biting the US in the ass? Yeah, most likely things like 9/11 won't happen or they won't turn out the same way if they do, but damn, knowing they did once and potentially could again, could give my new US a chance to do something about it. I'm sure as hell not going to sit on my thumb and hope some whackjobs don't start crashing planes into prominent buildings in twenty years or that the people on Flight 93 can manage to down the plane short of its target this time.
I'm pretty sure that every single person that ends up dead this time around because Spock didn't open his mouth will take comfort in the knowledge Spock feels good about himself at night and some directive which not only doesn't exist yet, but doesn't even apply to an alternate world scenario has been upheld ... oh, wait.
Spock Prime: Yeah, that Whale Probe that just ass-raped Earth Fleet and destroyed the planet, I totally knew that thing was coming, but figured you'd know how to stop it this time. Wait, the Vulcan Science Directorate doesn't exist anymore and they never figured out that special star slingshot timetravel maneuver and nuMe never went through recovery on Vulcan to learn it anyway? Ooooo. Sorry. My bad. Probably shouldn't have expected you to be able to pull the same Vulcan desert rabbit out of your ass twice when your ass is no longer my ass and Vulcan desert rabbits don't exist anymore cause Me and Nero fucked with the timeline. You'll get the next dire threat to Earth right, though. I have a really good feeling about the next one. Oh, right. Earth's dead already. Hey, we've got plenty of room on New Vulcan for your tattered and broken handful of survivor refugees.
