I don't think open exchange of people is useful, or ever has been. What has helped the U.S.'s "success" (in kicking ass, generally speaking) is one-way brain-draining of other countries.
There are many people in science who go to the US for research for a couple of years and then go back home - and people from the US going abroad to escape uncertain funding, enjoy new perspectives (CERN!) or the myriad of other issues that the US has (healthcare, discrimination, public schools, transportation).
