Distrubution of resources, now, during our collapse and after with regards to anything remaining while important, are secondary to getting all of humanity’s ecological footprint down to sustainable levels. This necessarily means fewer people AND less consumption.
That doesn't necessarily follow, and is inconsistent with past observations. At a micro level, take the example of greenhouse emissions from the United States, which peaked in 2007 and have come down since (despite population growth and economic growth). On a per capita basis, the United States peaked in 1973.
https://ourworldindata.org/profile/co2/united-states
At the same time, we simply cannot afford for other nations to increase their emissions to US levels on a per capita or per GDP basis. None of that has anything to do with the birth rate, and comparing the birth rates of different countries doesn't reliably predict whether their CO2 emissions equivalents change (either by amount or by percentage).
https://ourworldindata.org/grapher/co-emissions-per-capita
Simply put, the relationship between birth rate and effect on environment is so loosely related that pushing down birth rate is likely not going to push down pollution or environmental destruction. The solutions are actual engineering and economics, not family planning and demographic policies.
You don't need to dip into the negatives to show that one group of 1000 people consumes less resources than a group of 10 person. If personal resource consumption varies by several orders or magnitude between individuals, where one private jet trip over the course of a day can represent more than the annual consumption of someone else, then it is very easy to show that the correlation between population size and aggregate net resource consumption is weak.
No, running the same analysis by place of consumption doesn't significantly change things, because the biggest drivers of greenhouse emissions are still local consumption: transportation (especially air travel), heating, and things like concrete manufacturing (where the concrete tends to cure on site).
Ocean-based shipping is so energy efficient on a joules per kg (or per cubic meter) per kilometer traveled that shipping a container 10,000 km from Shanghai to Los Angeles uses significantly less energy and emits lower carbon emissions than a 1,000 km route over land.
My point is simple: anyone who believes that climate change is solved by depopulation is dead wrong. We should still be working to reduce emissions in places that have stagnant or dropping populations, because everything we've seen in the last 50 years (which you describe as a selective period, but I select that period because it's been the worst in world history for carbon emissions and climate change) is that countries significantly increase their resource consumption right around the same time they slow down their population growth.
You're fundamentally misunderstanding my point as an argument for the status quo, that what we as humanity are doing enough. No, I'm arguing that actually making the right changes are going to be orthogonal to population growth. Decarbonization is important, and needs to be done, even if you Thanos snap half the world's population, because there's nothing stopping the remaining humans from being even more resource hungry.