Nuclear war is not that big a deal. Destruction of a nation? Easy, no problem. Destruction of the species? Not even close.
Barring an act of super villainy, nukes would be fired at actual targets: At enemy silos to destroy their nuclear response, at military bases to destroy their military response, at industrial centers to damage production, and at population centers only if the intent is to kill people. The fact here is that even attacks on cities are not significant in long term, because cities aren't where you grow crops or get water. A nuclear attack would have to target EVERYTHING, with the purposeful intent of killing EVERYONE, to actually even have a tiny shot at it. In EVERY COUNTRY, from the great powers down to who-cares Madagascar.
To endanger our species, a threat must be both global and indiscriminate. A plague has a chance, although not a great one. Odds get better the more cosmic the scale, like an asteroid knocking the moon into Earth like a cue ball. Something messes with the planet, then yeah we're pretty much all dead, no chance for recovery, not much chance of even seeing it coming. Personally, I expect that global climate change will segue neatly into a new ice age, making trying to survive like living on the moon. As it becomes next to impossible to replenish food and heat, the population will trickle away to starvation and disease.
