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...
In a simulator, could you have 12 billion people with their needs met, living fulfilled lives and continuing into the far future? Yes.
Is there a political and practical way to reach that state? No. "One study estimates it would take just over 5 Earths to support the human population if everyone’s consumption patterns were similar to the average American." Any US government that tried to bring America's environmental footprint down to a sustainable or fair level would be voted out. It doesn't matter whether it would be gas taxes, meat taxes, per mile taxes, flight taxes, carbon tax and dividend, building renewable energy in less developed countries, or any other scheme. It doesn't matter if it was targeted at the ultra rich or the middle class. The sheer scale of it would cause Americans to vote out the government. And the same is true for any democracy and plenty of the non-democracies too.
We're not even vertically farming at scale yet.
There's a lot of room left for densifying human civilization. Seems like 10x should be achievable.
Why?
Why isn't 8 billion enough? Is there a plastic ring prize if we hit 80 billion?
Or are you talking about squeezing the 8 billion we have now closer together?
Perhaps consider that those 80 billion souls will contain Einsteins and Mozarts.
People are having fewer children, as living conditions, education, and acces to TV increases the reproduction rate falls.
Ideally we'll level out at 6 billion or so.
You haven't given any good reason why the earth should squeeze as many upright aquatic apes on it as possible yet.