Superpower status doesn't depend on population. The US is already the third largest country in the world in terms of population. We will have close to half a billion people by 2050. One in four residents of this country will be Hispanic.
Our energy needs, infrastructure requirements such as roads, sewage treatment plants, electrical grids, etc. will need to be expanded along with schools, hospitals, social welfare systems, etc. Our economy and standard of living could very well decline along with a Balkanization of the country along cultural and linguistic lines. And in 2030, we will have nearly 80 million people drawing SS and Medicare, about double what we have today. SS and Medicare are on the verge of going belly up.
China's population is not declining, just slowing down due to the below replacement level fertility rate [by design]. Thew world's population is increasing by 57 million a year [the equivalent of adding a new Italy] and will continue to do so until at least 2050.
India is growing faster than the US and will be more populous than China in the not too distant future.
Why is it such a problem if 1 out 4 Americans are Hispanic? We went through this route before and we did just fine.
The population is growing older even as it declines. The means a population less vigorous in every regard.