So it's not existential. That's what I've been saying the whole time.

Good grief.

A threat can be not existential and yet so bad that you should be willing make massive changes to avoid it.

(Edit: I just read a half dozen of the letters; none of them mention climate change as an existential threat. Rather, they express concerns of the type that I already have expressed. Which is not surprising, because this is what the science is indicating, as per the IPCC AR6 WGII report.)