America's biggest advantage over other nations is our immigration system, regardless of the myriad issues it has. The best and brightest are born all over the world, and they immigrate to America. Even if China stops immigration, we still have the other 6/7ths of the world to pull from.
Getting one started usually isn't a conscious policy choice. You have relatively better conditions to start with (no war, good location with stable climate, good civic institutions, limited immigration restrictions) which kicks off the virtuous cycle.
Here's an example. The Irish famine led to Irish emigration down the road, which benefited the US and Australia. This wasn't a policy choice of the receiving countries. It was mostly luck. These kind of relative differences in liveability between the two countries (US > Ireland) leads to a positive feedback loop in the US.
Realistically US draws from ~1.2B of English speakers. PRC draws from ~1.1B of Mandarin speakers. Actually pool of relevant STEM talent is obviously much smaller, but PRC generates about as much as all OECD countries combined, which is multiple times US can absorb via immigration unless (unpopular) shift in policy. PRC academic / S&T / R&D pipeline is maturing towards generating "best and brightest" in numbers comparable to rest 6/7th of the world, much of whom are already retained in PRC. And despite how many Chinese foreign students are in western universities, this isn't the 90s anymore where CCP actually sent out their best and brightest to learn abroad, most of the international PRC students now are those who can't hack it in high competitive PRC system but have the wealth to pursue education abroad. They're extremely competitive bench warmers. Which isn't to say US brain drain isn't a great competitive advantage, it just may not be disproportionately so, something that keeps US in the game instead of thoroughly dominating it.
But it seems obviously bad if someone goes from China to the US for a phd in some critical field and wants to stay in the US (either for better opportunities or for the relationships they have developed or because they like living in a free country) but is forced to return to China by the US immigration system. Maybe phd students in stem in the US also bias foreign because there are just more non-Americans than Americans in the world or because American citizenship allows natives access to better opportunities (eg getting paid a lot more money in tech or just having a job where you get some level of support from your superiors).