> But a lot of skilled labour left anyway. Partly because the general atmosphere got unpleasant. But also highly paid people have spouses, children, parents and other relatives. Once you are told you barely cleared (very high) criteria, you can be pretty sure your retired parents won't, if ever you need them to move in with you.

Chain migration is an anti-immigration argument in the US - even if some particular immigrant is highly-skilled and would benefit the US from being a legal resident, that immigrant's family is probably not as impressive. Nonetheless, once the government gives legal residency to the highly-skilled immigrant, they will be highly motivated to try to get the rest of their family to the US as well. So the US should be careful about granting legal residency even to prospective immigrants whose credentials make them individually look good.