People here don't understand the difference between total population and useful population. Useful population is the lower middle class and above and those with education of course. Total population includes villagers living in dirt floor huts, doing basic farming producing barely what they need to survive (sometimes less), with nothing really taxable. You can have a trillion of those and your country wouldn't be able to challenge anyone. China has a (slightly) declining total population with a rising useful population, so its economy is well on track to continue growing as it always has.
In the past, the rural poor in China provided the labor pool for China's rapid development, industrialization, and urbanization. They were responsible for building China's cities, infrastructure, and factories, as well as working in the low wage manufacturing jobs that generated the initial capital needed to fuel China's rapid rise. It is imperative that this labor pool does not dry up before sufficient advancements in automation are made that would drastically reduce the amount of unskilled labor required. Otherwise China will be in the same situation as the US, where labor costs are so high that it becomes prohibitively expensive to build anything.
You still need people for the demand side of things. China doesn't suffer from this nearly as much as it's population is still massive even it it's slowly declining and the population as a whole is getting richer , so consumption/GDP will still rise at a steady rate for a few more decades. But eventually wealth and the amount of things a people can consume maxes out, so the main way to boost GDP is via population growth, we sort of see this with America already. And of course eventually population decline is gonna hurt in around 50 years if China doesn't boost it's birthrates, right now it's basically a non-issue, but when your average age is in the 50s, it's not a joke.
Maximizing consumption, both on a per-capita basis and especially on an aggregate basis should not be the goal. It is not sustainable to promote a massive population consuming as much possible. China's per capita consumption is currently low so there is room for growth, but only up to a certain point. The goal should be a sustainable population with maximal quality of life for all and where you have sufficient human capital to produce/develop everything that is needed. Gradual decline in population over the next few generations would be ideal. The problem with a sharp drop in birthrate is that eventually the proportion of working age people becomes too low, giving rise to the situation where each working couple has to support 4 elderly parents as well as (ideally) 1-2 children. Policy should aim to push birth rate up to around 1.6-1.8 ( and eventually 2.0 once the desirable population level is reached).
China itself will be struggling with climate change in southern China. It will be interesting to see if the 2 decade population decrease of northern China can be reviewed. Everything north of Henan was declining in population except Beijing.
I suspect that border provinces with similar minorities will be popular migrant destinations from Southeast Asians i.e. Vietnamese in Guangxi and Burmese in Yunnan.
Newly urbanized Chinese maybe will migrate to central and northern cities like Xi'an, Zhengzhou, Shenyang, etc. especially with cheaper housing and government jobs. Lots of little problems can be overlooked with cheap houses, money, and everyone speaking Mandarin.
The problem is that a lot of the climate change is actually impacting the north. Desertification and water shortage are already significant challenges in North China, so I doubt that North China outside of few select areas in the Northeastern 3 provinces can support any substantial increases in population. The most comfortable and pleasant region to live is by far Jiangsu/Zhejiang. Once you've spent any amount of time in a nice Jiangnan city, you will not want to live anywhere else (in the world). Not only the is the climate ideal, the cities in that region tend to be the best developed, most advanced, cleanest, newest, most modern cities in the world. Even BJ falls far behind cities like Suzhou/Hangzhou/Wuxi/Shanghai.
Also maybe there will be mass migration from Mongolia to china than Vietnam due to desertification that Mongolia can't control.
The population of Mongolia is roughly the same as that of a single district in Beijing.