If you compare https://www.populationpyramid.net/china/1980/ with https://www.populationpyramid.net/china/2020/, you see how drastic its effect on China’s population pyramid was. Drastically fewer kids means fewer parents 20-30 years later.
(There also is a bit of an effect of richer people having fewer children, but for China, that effect is relatively small)
Can you clarify on what you mean by that please? Are you referring to pollution an environmental degradation?
Think of the percentage of any modernizing economy that goes into things like food and clothing for children, education and educational materials, transport, energy consumption. The reduced output of parents who are tired from looking after or arguing with children. And this is for something that won't start putting value back into the economy for a good 18 years from initial investment.
Skip this, and instead of food and clothes for children adults can spend on housing, transport, take risks with starting businesses without the chance of hungry dependants, etc
1B feels like a lot more realistic estimate than 750M.