And that taboo is probably rooted in evolutionary psychology, people have a genetically driven tendency to criticize those who advocate having less children? So could there be an instinctual drive behind it?
https://www.flashpack.com/solo/relationships/dont-want-kids-...
This has been studied long time ago by scientists such as Alfred Sauvy [1], who concluded that overpopulation is not the cause of sustainability crisis, and that greenhouse gas is the major cause. In particular, limiting the growth of population has few impact on the production of greenhouse gas, whereas changing the means of energy production and consumption is much more impactful.
Moreover the world population is expected to be less than 12 billions in 2100 [2], which is plainly sustainable. This is mostly due to the demographic transition, a pattern observed in most countries, where the fertility rates decrease over time. More specifically I recommend the excellent book of Emmanuel Todd and Youssef Courbage on this subject [3]. The authors argue that in most countries throughout history, when both the majority of men and the majority of women know how to read and write, then the fertility rate decreases, and a revolution becomes imminent.
[1]: https://en.wikipedia.org/wiki/Alfred_Sauvy
[2]: https://www.un.org/en/desa/world-population-projected-reach-...
[3]: https://cup.columbia.edu/book/a-convergence-of-civilizations...