Do you hear that sucking sound? It's biggest brain drain you've ever seen. It already been happening, I know several great scientists who've already left, this BS just kicked it into high gear.
Back to their home countries. I can think of about a half dozen world leading scientists I work with every day who would pack up their labs if funding dried up. They already did it once to move to the US because that is where the most funding was for these diseases. If you make the US inhospitable to these talented people, they will not think twice about finding a country that doesn't consider them "bottom feeders".
Also. Believe it or not, there are other fields than DEI/Climate/Gender studies, and when you remove the incentive to work in the US, the leaders in these other fields will leave.