Earlier quoted context omitted.
It's very simple. If it's harder for foreign people to become postdocs/profs in the US, the quality of US research will fall. Quickly.
Citation heavily needed here. I find it hard to believe on face value that foreign researchers are necessarily better than US researchers; especially given the number of both foreign and domestic university graduates from U.S universities is much less than the available postdoc and professorship appointments in STEM.
But you have to also remember that the current group of academics came out of PhD programs in the 1970s. That was a time of expansion in academia. These folks are just about to retire. On the other side of this, I think the universities walk a fine line with respect to graduate student admissions between foreigners and US citizens.
The fact that we bring in foreign graduate students from foreign undergraduate programs either means that our current system is not producing applicants of sufficient caliber who then apply or it is not economically viable. In that sense there is evidence that graduate student research stipends are depressed. Is this due to foreign competition?
There is some anecdotal evidence to this in that the majority of CS undergrads get a job ratherhe than pursue a PhD etc... b/c of the salary differences. Foreigners have to get a masters at a minimum to achieve better odds in the H1B lottery. And after they graduate from a US masters program, they can work at a US company for a couple years anyway.