And the present system means that Americans are almost entirely blocked from pursuing careers in science by the awful economic prospects. Foreigners that come here can always get good appointments back home that will support a family if they can't get tenured in the USA. Americans would be out of work with zero savings and mediocre prospects at 40.
If you think Americans have something valuable to contribute to the world of science beyond than our cash support, then you can't support the level of mass foreign migration into science schools and professions.
Why don't Americans, discouraged by the economic prospects of our careers here go to foreign nations and support a family? Foreigners are willing to uproot themselves to start in the United States. And we are all better for it.