I find it fascinating and terrifying that on the one hand, these events were "close calls", on the other hand, considering that we can celebrate this event today, they were inevitable.
Logically speaking, we always must live in a world where past existential threats to humanity were avoided (including super-volcanoes, asteroids), but that says nothing about the possibility of future existential threats wiping out humanity.
Spoiler (including why it's related to your post) since I think it's easy to miss what this is: Vg'f n obk gung, jura bcrarq, qrfgeblf gur havirefr. Fvapr nccneragyl guvf obk yvirf va n znal-jbeyqf havirefr, vg'f onfvpnyyl n havirefr-fpnyr Dhnaghz Fhvpvqr qrivpr.
And yes, I'm extrapolating a bit, but given the rest of the SCP-universe (inasmuch as there is a such a thing), it's a pretty reasonable conclusion. If it's wrong to you, well, it's right to me, Hail Fanon, All Hail Discordia.
A war would have thoroughly wrecked civilization, but plenty of people would have survived, especially in third-world countries.
There's no reason a much diminished humanity concentrated in the southern hemisphere couldn't be marking the thirtieth anniversary of War Day today, if things had gone a bit differently.
Indeed. If you want to find out more, the best book that I've found on this subject is _Global Catastrophic Risks_, edited by Nick Bostrom and Milan M. Cirkovic:
"In Global Catastrophic Risks 25 leading experts look at the gravest risks facing humanity in the 21st century, including asteroid impacts, gamma-ray bursts, Earth-based natural catastrophes, nuclear war, terrorism, global warming, biological weapons, totalitarianism, advanced nanotechnology, general artificial intelligence, and social collapse. The book also addresses over-arching issues - policy responses and methods for predicting and managing catastrophes. "
Logically speaking, we always must live in a world where past existential threats to humanity were avoided (including super-volcanoes, asteroids), but that says nothing about the possibility of future existential threats wiping out humanity.