I hate to say it, but you've got it exactly backwards. QoL is much better after population crashes. I'll give two examples and explain the simple reasons.
For example, life in the early 1300s in Europe was awful. Life in the middle 1400s was much better. Reason: The Black Death killed a huge number of people, opening up quality land for people to live a bit more easily.
You don't want to live during the black death, you want to live after the black death. Because the black death can only happen above a certain population density.
Micro-example: Pitcairn island. A handful of men and women shipwrecked there in the 1700s. Life was good - go where you want, lots of space, pick wild fruit as desired. Then they multiplied. Soon the population is 10x as much, and they hit the carrying capacity of the island. Now you MUST use every awful marginal source of food.
The simple reason is: People exploit the best resources first. The first person in an ecosystem lives in the best spot, eating the best food from the easiest tree, pulling fish casually out of packed rivers. Paradise. Increase the population, and now most people MUST live on marginal land, eating marginal food sources, scraping every bit of sustenance out of the environment. QoL falls until it is just barely above what's necessary not to die - and then you're at carrying capacity and the population stabilizes because it can't grow because people die as fast as they multiply. This is how human life worked for almost all of history; it's how animal life works too.
It explains why QoL was the same in 1800 for most people as in 2000BC, despite great increases in technology. The technology increased QoL, but that gets consumes in one generation as the population increases to hit the subsistence limit again. So you get more people, but not better life.
They are still really deadly diseases for vulnerable populations, but I can't even imagine what it was like when they ravaged populations.
You reduce child mortality (and improve other measures) by freeing up minds from menial labor through economies of scale. Those free minds invent processes that ease human endeavors and lower humanity's impact on the environment in drastic ways.
These aren't small gains. The history of artificial light is an example, showing improvements in efficiency in magnitudes of tens of thousands.
We still have a lot of work to do to protect whales, but there's a reason Moby Dick resonated when it was written, everyone knew about the significance of that then major industry.
I'm not a transhumanist, but I am awestruck by the pace of human progress. Solar adoption looks glacial while we're waiting for price parity, but is there any doubt it will displace fossils over the next few decades?
Human inventiveness seems the best way to lower impacts while also obtaining other human goals. For all that I want the highest MIPS per gram ratio in the solar system as possible.
For example, life in the early 1300s in Europe was awful. Life in the middle 1400s was much better. Reason: The Black Death killed a huge number of people, opening up quality land for people to live a bit more easily.
You don't want to live during the black death, you want to live after the black death. Because the black death can only happen above a certain population density.
Micro-example: Pitcairn island. A handful of men and women shipwrecked there in the 1700s. Life was good - go where you want, lots of space, pick wild fruit as desired. Then they multiplied. Soon the population is 10x as much, and they hit the carrying capacity of the island. Now you MUST use every awful marginal source of food.
The simple reason is: People exploit the best resources first. The first person in an ecosystem lives in the best spot, eating the best food from the easiest tree, pulling fish casually out of packed rivers. Paradise. Increase the population, and now most people MUST live on marginal land, eating marginal food sources, scraping every bit of sustenance out of the environment. QoL falls until it is just barely above what's necessary not to die - and then you're at carrying capacity and the population stabilizes because it can't grow because people die as fast as they multiply. This is how human life worked for almost all of history; it's how animal life works too.
It explains why QoL was the same in 1800 for most people as in 2000BC, despite great increases in technology. The technology increased QoL, but that gets consumes in one generation as the population increases to hit the subsistence limit again. So you get more people, but not better life.