In the 2018 Bloomberg Innovaton Index I can't see any obvious correlation between list position and levels of immigration - though that's not to say the UK's ranking (17th...) might not be even worse without overseas academics, but clearly other factors are at play.
The UK's position would be significantly worse, if it hadn't woken up in the 1980's to the "brain drain" and done something positive about it to encourage UK resident academics to remain in the UK by both increasing funding of UK university-based R&D and promoting the UK as the place to do R&D. So I think in this specific case, the UK hasn't so much imported overseas academics in the same way as we import nurses etc., it has created an attractive environment for interesting R&D, which has attracted both investment and people.
I'm not sure that returning people to places to which they've had no connection for decades can be regarded as anything other than xenophobic vindictiveness.
Well it really is up to the individual to make the appropriate applications to have their 'home' officially recognised as being the USA. I suggest people don't because they regard the "places to which they've had no connection for decades" as their "permanent home". Hence all that is happening here is that the authorities are telling those who have made the decision that the USA is their "temporary" home (even if it is for decades) that they are no longer welcome and should return to the place they clearly regard as "home".