Germany had about 65 million in the 1930s, before WW2 and it's annexation binge.
That increased to 80 million because of migration. Yes, you can always import more people from countries that have a lower standard of living than yours. Importing people from countries with higher standards of living is not so easy. And there are significant long term costs to that approach as well as benefits, especially the costs of integration, which is often very hard to accomplish, as well as ethnic strife and racialization of politics. But migration is not something that China (or most Asian nations) are interested in. They will need to maintain their population organically. And I really see no reason why places like China or Japan would be worse off with fewer people. Once the population drops, land becomes cheaper and those less likely to have kids exit the gene pool. Things stabilize at a lower level where cities are less crowded. Declining population is a sign that the society isn't conducive to reproduction and it comes with its own solution of population declines until things are brought back into balance.
That increased to 80 million because of migration. Yes, you can always import more people from countries that have a lower standard of living than yours. Importing people from countries with higher standards of living is not so easy. And there are significant long term costs to that approach as well as benefits, especially the costs of integration, which is often very hard to accomplish, as well as ethnic strife and racialization of politics. But migration is not something that China (or most Asian nations) are interested in. They will need to maintain their population organically. And I really see no reason why places like China or Japan would be worse off with fewer people. Once the population drops, land becomes cheaper and those less likely to have kids exit the gene pool. Things stabilize at a lower level where cities are less crowded. Declining population is a sign that the society isn't conducive to reproduction and it comes with its own solution of population declines until things are brought back into balance.