The climate catastrophe is caused by a hyper-reliance on fossil fuels & deliberately shitty transport infrastructure (i.e. the private automobile & it's consequences), entirely for financial reasons; not just raw numbers of people.
You need people who can actually do work to take care of all the old people & sustain human society. "Less People" is not by-default "more sustainable" especially not if it happens all at once; that was in fact a huge problem with cyclical famines & political turmoil in the days before mechanized agriculture.
If some asshole went around raiding hamlets for plunder, or whatever reason, yeah that would mean fewer mouths to feed in that particular area, but it also means fewer hands to bring food to harvest. Which means other regions have to contribute larger proportions of their own food-stock to sustain the needed intake of urban centers. Which means that they have less food to eat for themselves, and less to replant for the next harvest. Which pushes people on the margins of the the agricultural economy into banditry to sustain themselves, which causes us to return to the beginning of our story.
Eventually this cycle of regional depopulation leading to productivity shortfalls, leading to further regional depopulation becomes self-reinforcing & before you know it you have a country-wide catastrophe on your hand & the total implosion of existing society.
Now we aren't dependent on mass manual agriculture these days, so famine specifically is an unlikely cause of cyclical societal collapse, but the modern world still requires that a shitload of manual physical labor get done in order to maintain the basic infrastructure that gets everything from where it is, to where it needs to be in order for us to all not die. If you don't have people to fill those positions, then that's work that needs to get done, that isn't being done.
Why should it?
That's asinine, you're treating periphery countries like they're glorified breeding-stock for the developed world's work-force.
Edit: To make my point more clear, the whole reason why developing nations have higher birthrates than developed ones is because they're developing/underdeveloped. They lack access to contraceptives, and substantive access to women's healthcare; and they also oftentimes have economies that still rely to some extent, or a large extent on non-mechanized smallholder, or subsistence agriculture. That, or they otherwise have social institutions that allow for, or require children to enter the workforce. This means that having children in those countries is often an economic boon to a family (because they can contribute to household incomes through work), and avoiding having them can be very difficult for women.
If you solve their problem of being underdeveloped, & hyper-exploited (which you should be doing if you're a "queermunist"), then that means that they are likely also going to be in a position where they have declining birthrates because there will no longer be an object material incentive to have children, and women who don't want to would be able to prevent it.
The idea of shoring up a declining population "through immigration" only works so long as you have an underdeveloped periphery of peoples who want to come flock to the West, or to developed nations in search of higher wages & a higher standard of living (or just avoiding Imperialist political meddling), rather than staying at home.
That is a saltine stick.
It's bald pocky.
START NOW! You need those!
Well, I see what you're problem here is.
You didn't build any coal for steam-powered pumps, you goofball.
I know what the acronym is, I'm asking how in the goddamn do you figure it has them?
You are a bundle of atoms destined to lose cohesion, revel in the beauty of it.
I don't see beauty in any of it though. It's mostly ugly, and gross.
Gotta eat your veggies if you want desert.
Actually, he thought it was going to be arthropods.