The question is - how can be free up a higher percentage of people who are able to propel humanity forward from the drudgery of wage slavery in a sustainable manner?
And you need to effectively allocate cognitively proficient people to this as well, not just anyone.
1. Rapid change of any kind is bad in any ecosystem. Growing the population too fast creates one set of problems, shrinking the population too fast creates a different set of problems.
2. modern humans have the ability to expand consumption to use all available resources. 10 million jet-setting billionaires have a far worse impact on the environment than 1 billion poor people. Adjusting the environmental impact per human is going to have a far greater effect on the environment, and more quickly, than population changes.
And you are right - but poor people _want_ to have material luxuries, by and large, so we should consider what we want our equilibrium state to look like.
Personally, I'm hoping for "8-10 billion people eating low meat diets, living in well insulated homes powered by renewable electricity, in places where they can walk, bike, or take public transport for almost all of their daily needs, and working a 20 hour (or less) week, with the remainder of their time filled with their friends, families, creative passions, and any other joys not yet comprehended"
but it doesn't seem like that's where we're headed.
I’m not the original commenter, but I assume that the reference is that this/your group fairly reliably across most cultures does not maintain the 2.1 children per woman birth rate / replacement rate needed to sustain a population.
At a minimum, this seems to be true for this particular socioeconomic class in most parts of Europe, East Asia, and US/Canada. Not sure about South Asia.
We are not on track for equilibrium or a slow decrease over centuries. We are on track for a demographic cliff.
From individual families' perspectives, you can only have a whole number of kids anyway, so 2.1 vs. 2.04 isn't a meaningful difference.
But given that the statistics have wide error bars, and there are so many other intervening factors, "a bit more than two" is just as good a way to calculate it.
I like people! I have two kids myself and love them. But it's not like life was horrible when Earth had 5 billion people.
(Note - I am NOT saying that people in poor countries, who tend to use a minute fraction of the resources of rich people, having lots of kids, is necessarily an issue. But if we want to make a higher standard of living possible for everyone in the world it will be easier to do, ecologically speaking, with 9 billion people than with 15 billion)
You won't have many brilliant people in a small natural population.
Without them it's «not with a bang, but with a whimper»