Why do you have the maximalist outlook that population must grow. All developed nations are looking at population crunches. All of them, and China and Russia as well. Reducing it to a Chinese problem is reductive. It’s a collective choice, and how is China doing better or worse than its peers?
It's probably good for the planet if we depopulate. But the problem which comes with that is that socioeconomic policy and infrastructure has massively evolved over the course of the industrial and digital revolutions, boosted in the US by WWII, etc. A sudden decline of population makes things that were touted as viable or stable suddenly less viable or stable.
For example, the dependency ratio changes, especially in an aging population. Look at what Japan's going through. A working married male in Japan might be taking care of both their immediately family and both their parents and their significant other's parents. It's a significant economic load and leads to significant issues around mental health and work-life balance.
We can also look toward Japan as a test bed too, as their GDP and standard of living does continue to rise despite an ongoing population decline. This is not an impossible situation to manage, but it does require strong and thoughtful leadership
There's also the lost of trade skills and workers in general needed to maintain current service-based infrastructure.
In the case of China, their population of nearly 1.5bil is projected to be halving within 75 years. This is a massive difference that will require recalibration of policy and infrastructure, whereas other countries might experience a significantly lower ratio of decline.
I don't, I'd be perfectly happy with a stable population too. Infinite growth is a dumb goal and I don't support it as a goal (at least until someone demonstrates space colonization).
But a society where generations get dramatically smaller is screwed. You need a balanced (or better) ratio of workers to retirees, mainly. If you took the US and teleported a representative (by age) sample of 80 million people by age to another planet, our society here wouldn't collapse, everything would be pretty ok, we would still have the same basic economy, and the fewer kids and elders to take care of, the reduced consumption, would balance out the loss of the workers. But if they took 80 million people between 30 and 65 instead (that's probably most of that age group), things would go really badly for us, because there aren't enough 20-somethings and 65+ workers to do all the things that keep the kids and retirees fed. If we simply wait 30 or 40 years, China (among others) will be in a situation like that. US is not much better and I can certainly imagine it getting much worse in the US. And I can confidently predict a maximum number of 30 year olds we'll have in 30 years because every one of them has already been born. Even extreme measures can at best only make more 2055 29-year-olds.
> how is China doing better or worse than its peers?
As I stated, China has one of the worst birth rates -- numerically it's doing worse. I don't have a beef with China. I hope they and the rest of the West can figure out how to improve the birth rates soon.
