Even if it were only one in a hundred foreign students that turned out to actually be exceptionally talented, taking them all is a drop in the bucket relative t

Even if it were only one in a hundred foreign students that turned out to actually be exceptionally talented, taking them all is a drop in the bucket relative to overall incoming immigration, both legal and not. (Even if the ratio were one in a thousand, it would still not be all that bad; the STEM student population is not all that large.) Moreover, even if most foreign born students in a STEM program were merely as good as native students, as you suggest, that still puts them intellectually and educationally well above the average citizen, let alone the average immigrant. There is no way that it does not make sense to offer foreign STEM students green cards, so long as the university is reasonably accredited.