question of who turns to capitalism or communism

Disagree. The lack of resources means economic de-development. Calling your economic system either capitalist or communist at that point is arbitrary (yes technically they could be categorized but it is of no practical use). The only exception I see are nations like China and Russia that have vast resources that can also defend themselves. And even they will face a decline as they adapt

the systems in place don't exist on the basis of informed consent on the part of the whole population or an even more nebulous mechanism that decides what is "effective," it is based on the degree to which the proletariat is organized as a coherent political force and the extent to which the country is developed such that it can actually support socialism

First this is just a truism. Secon, What's your point because next you say

Neither of those things are the case almost anywhere in the world that isn't already calling itself socialist (and even in some places that are).

Is it that countries won't go socialist? I mean I don't think socialism is going to thrive in the capitalist carcasses of most counties once capitalism fails. It could, you never know but I don't get your point. You aren't making your conclusion clear at all. Like I dunno maybe give your opinion on how things will play out.

You've shifted from pure geopolitical wishcasting

Do you know what wish casting means? Why would anyone want things to play out like this?

And I have no idea what you mean by environmental collapse posadism. I'm sorry to address each point individually but I can't find your overall argument so here we are