There are two different groups I think. One is not having kids because of environmental issues, but I think there's a larger group not having kids because of systemic societal issues that make it hard to have kids. (sometimes it's the same group but not always)
So part of what I was pointing out is that when people say "we need less people" they aren't implying "wow, I'm glad the population is shrinking" they're really saying, "less people need to be born in the future"- which probably isn't true.
> and the population is an obvious lever to scale that impact
Also, it's not obvious, since, with the exception of China, no one actually meant to do this. It might actually lead to a lot of human misery, the same as climate change might do, so it's not clear that having less people in this specific way is a net good- for example countries like Japan completely collapsing in on themselves and going extinct. I think what you meant by smaller population was just have the same exact society but thinned out, but that's not what this next chapter of degrowth looks like. The boomers who insist on their full pensions at 60 are pretty much killing the youngest generation and leaving them with nothing, and to a certain extent everyone is being rational actors- there's no reasonable way out of the problem.