Status: speculative: I thought I would throw this out there for feedback and maybe start a brainstorming session for any other related ideas.
Twelve out of 16 (Allison 2017) times that there has been a switch in which is the most militarily powerful country in the world, there has been war (though we should not take that literally for the current situation). Still, it might be worth investigating ways to prevent that switch, especially if they have other benefits. The size of the economy is not the same thing as military power, but there is a correlation, so I will focus on the economy.
China’s purchasing power parity is already larger than the US, but China’s market exchange GDP is still significantly lower than that of the US. The question is whether increased immigration to the US could prevent China’s market exchange GDP from exceeding that of the US.
As Carl Shulman points out, just one large developed country opening its borders could absorb billions of people if it attained the population density of South Korea.
This shows that peak immigration to the US was about 4.5% of the US population (and ~0.5% recently). By the way, it has an impressive animation of immigration to the US over time.
One estimate of the GDP growth rate of China in recent years has been falling from ~8% to ~6%, and for the US ~2%. If one were to naïvely assume that GDP would scale with the population despite significantly larger immigration, one would estimate that if the US returned to its peak immigration rate, the GDP would grow about as fast as the Chinese GDP is growing now. Of course that large of immigration rate would likely reduce the GDP per capita growth, but I think the expectation is that China’s GDP growth will continue to slow. So I think it would be feasible for the US market exchange GDP to stay ahead of China’s. This would be made easier if a lot of people from China came to the US, which I think would be a generally good thing in terms of US-China relations.
I go won’t go into the long list of pros and cons of increased immigration, but I will sketch out some thoughts on this particular motivation for immigration. Perhaps the argument of reducing the chance of great power conflict would persuade some more people to support immigration to the US. Another argument is appealing to national pride of not being overtaken. But considering this particular argument, it does seem a bit strange to promote nationalism in order to reduce the chance of conflict, so could this backfire?
Edit: A new book called One Billion Americans makes a similar point (I haven't read it).