For numbers, it's nearly 1% of all post-docs (~3400) in those three universities, leaving at once, to a single destination. You can do the math. It's a fact that the USA used to attract this talent, not export it.

Academics have always been leaving the US, just like Academics have always been coming to the US. A turnover in staff is normal, it has happened everywhere, always.

>It's a fact that the USA used to attract this talent, not export it.

And where is the evidence that it does not? People leaving is normal. Post-docs leave institution's all the time.

One additional thought. If you think this is about a right wing political shift in the US, why would these researcher go to one of the strongholds of the far right in Europe?