It's true that China will keep growing in financial power, but
it's not true that people will want to migrate to be a Chinese
citizen. The human rights abuses are still happening, the place is
still mostly in poverty and the govt is still a dictatorship.
Moving there for work is one thing, and hundreds of thousands, if
not millions, of people already do that. But if you mean
citizenship, then no, that wont happen.
As for the US being supplanted, that wont happen either. The US
is one of the most popular countries that most people
want to migrate to. It remains a very rich and technologically
advanced country where you have the freedom to live, think and say
what you want it. This makes it highly attractive to most
migrants.
The Economist reports that a US think tank thinks it is possible
the population of the US may become as much as 500 million by 2050,
and almost 1 billion by 2100. Very large numbers, to be sure, but
no one denies that the US is much more attractive place than
China.
Many people can contemplate becoming American. No one considers
wanting to become Chinese.
Living in China is one thing. Becoming a China citizen is
something else altogether.