It all boils down to what hides behind that word, "existential". Slavs and Jews definitely considered what happened in WW2 an existential threat for instance, but you seem to be arguing that only threats to the entirety of humanity qualify as existential, a view that many others here don't agree with, probably because it gives off the impression that things like the extinction of entire cultures and millions or billions of people are discounted by using a restrictive definition. It seems however that neither view matches the use of the concept of "existential risks" in contemporary discourse, which are understood to be "risks that threaten the destruction of humanity's long-term potential". That's a much lower bar to clear and by that standard, we're clearly collectively cutting the branch we're sitting on, though probably not indeed jumping off the cliff of an absolute total annihilation of the human race.