When your sources are 1960s films, it isn't very persuasive. I haven't seen "Fail Safe". I have seen, but not read, "On the Beach". Let's not forget "Doctor Strangelove".
In some ways, it's a quibble. If 9 out of 10 people in the world die, it really sucks, for the ones who die and the ones who survive.
If 99 out of 100 people die, well that's even more death, which is bad, and a very interesting existence, although ripe with possibility, for the survivors.
But I haven't seen any reason to believe that nuclear war would kill 100% of the people. 100, or maybe 200 or 300 years after darned near everyone dies, the world will be back to worrying about overpopulation, because the way to correct for overpopulation will be in their history books, and those books will say that it sucked.
From what I've read, though, even a full scale, all out, throw everything you have at it nuclear war, probably wouldn't kill on that scale. Hundreds of millions dead within a week. Lots more dead owing to the collapse of infrastructure everywhere. The general consensus outside of pop culture is that nuclear winter wasn't really a thing likely to happen, or at least not likely to be an extinction level event. So, anyway, lots of people die, civilization collapses temporarily, lifespans are short for a time because of high cancer rates, and then the ones who are left rebuild, probably no wiser having grown up surrounded by ruins of great civilizations.
