I know several people on various visas that are making plans to leave after having gotten PHDs here. Still more have naturalized and are making contingencies for exiting.
Immigrants found close to half of the fortune 500 businesses, and start something like 20% of our new businesses each year. For those motivated immigrants to choose elsewhere is going to reduce our growth both from what they don't do, and because immigration is what has delayed our demographic inversion that Europe and other developed nations are going through.