About the demographic collapse from the One Child policy, I'm going to volunteer an alternative analysis, since you've asked for it, copied from another of my comments:
The effects of the One Child policy has actually been pretty much as intended.
There are two hidden goals of the One Child policy that aren't communicated very often in the West.
The first is proletarianization. Indeed, almost 30% of Chinese people are peasants. This is not efficient. Only around 2-5% would be sufficient; therefore 20-23% of the productive potential is wasted for the same consumption. And so, creating a situation where it would be possible to shrink the size of the peasant class would allow for the productivity per capita to rise, thus allowing for better surpluses and better living conditions.
In this view, a demographic fall is not an actual issue, because there is already 20-23% of "dead-weight" that can be eliminated.
Secondly, the issue is of human capital. This is not so well-known, but the retirement age in China is extremely low. Indeed, the average retirement age is at 58 (!). This is done on purpose so that the younger generation may enter the job market faster and learn more, reducing unrest and permitting the investment in human capital. However, this comes at the cost of enforcing early retirement, which also makes for reduced efficiency.
The solution to both of these problems can be to simply reduce the population, this will allow for a sharp decline of the peasant class and for an increase in retirement age.
In this view, the demographic effects of the One Child Policy, while they might have been disastrous for a developed, urbanized economy, actually fits into the demographic and productive plans. It is thus an incorrect to call them an error, and there will likely not be an economic collapse in response.