I don't even know what you mean by "let those kind of people go". If they can't get funding, they won't become scientists in the first place. If they lose funding, the US can't just prevent them from moving overseas. It's (still) a free country! And a large proportion of them are foreign-born anyway.

Geoff Hinton couldn't get a job doing AI research in his native UK, so he moved to the US, where there were a lot more opportunities. At that time, neural networks weren't seen as particularly promising. Decades later, it paid off big. That's the kind of thing that will happen less and less (or work in reverse, with US researchers taking jobs overseas) since there will be far fewer funding opportunities.