I hear this type of statement often, but people rarely mention the scope or who the brain drainees are. In my experience, it's exceptionally rare that American talent comes to Europe compared to the opposite, and I see little reason why that would change in the near future. When it comes to Chinese individuals returning to China from the US, this isn't exactly traditional brain drain, and it's also something China has actively, sometimes aggressively, been pursuing the past decade or so.

China has just developed to the extent that top STEM professionals can have a good quality of life at home. Chinese immigrants were pouring when the US was far less welcoming. The US used to also get hordes of migrants from Europe, Japan, Taiwan, and South Korea before they developed.