China's best brains still are going abroad.
nth proof:
Quanta Magazine picked up the top 3 maths breakthroughs for the year. Total 7 mathematicians, 3 Chinese.
All of the 3 chinese researchers are however in the USA. I have seen same trend in multiple different fields.
This isn't "proof" of anything.
1. It is undeniable that China's talent retention is superior to what it was in the past.
2. Quanta Magazine in NYC based; their picks are susceptible to bias just like Nobel prize picks.
3. China's rise and overtake of the West in technology is also undeniable; that simply cannot happen if China's losing all of its best talent to the US.
4. China's best brains may not be as public as those in the US. Whereas a Chinese gem may be hidden in a lab working on key technologies to foward the country without much fanfare or publication, one such Chinese person in the US might be a) there for selfish personal glory so they prefer to take up highly visible positions and roles to build personal brand and b) not be entrusted in extremely sensitive areas anyway.
I acknowledge the issue and would definitely like to see all Chinese talent unite; it would be the end of Western power. I believe that China is working quickly in the right direction to achieve this. But given the big picture of China's rise and the relative decline of the US, I do not believe the issue is nearly as profound as you say.