I really don't see the problem here? It's not like having a large population is in itself a noble goal that makes life better for people.

We don’t know how to run economies without young people. Look at the demographic issues in Japan, and lesser extent now in China.

"Cumulative emissions from descendants, decreases substantially if national emissions decrease."

So doing all the other ones actually lowers that one.

And even then it only makes sense if no-one else has more kids to compensate, so it's a personal action that will cut your personal carbon, but like most things in this area not something that works unless we all do it (globally)

Why would we have to reduce the population below a number which is sustainable? Nobody is advocating that.

The actual number could be fine tuned through tax breaks. Give childless couples an X% tax break. Single child families a Y% tax break. 2 child families no tax break. 3 child families a tax penalty. 4 child families a larger penalty.

Only if you assume the average is two children per couple and a replacement reproduction rate, which doesn't fit the reality I know. We have a huge population in the billions precisely because that's not what is going on.

If we drop to something below exponential growth rates, this is not a tragedy.