truly existential event

I now get what lead to this confusion. You've referred to both "existential" and "major civilizational-level catastrophes" without much effort to distinguish between the two, though they differ in both extent and probability by a few orders of magnitude. I assumed from the Bostrom paper citation and the long list of existential threats that the article in general was about existential risks, which, on a rereading, it isn't.

My concern over showing that something could reasonably be done remains, but you do provide appropriate evidence regarding civilization-level catastrophes. It might be worth a sentence or two clarifying that your concern is civ-level or greater, rather than specifically existential, though I may be the only one who misread the focus here.