The main thing that has made America exceptional has been that we attract all the smartest and most ambitious people from around the world. They work side by side by all of the smartest and most ambitious people born in America. We have won by having a disproportionate percentage of the world's smartest and most ambitious people, those born here + those who choose to come here.
So now, we're just throwing that all away and becoming a merely average country with an average amount of smart and ambitious people. Throwing away our key advantage for literally no benefit.
The smartest and most talented can still come, there are specific visas and pathways for them.
Also, let's not be as childish as assuming that policies that were good 100+ years ago for a country should be good ad vitam eternam. Countries change, their interests change, therefore their policies should change accordingly.
So now, we're just throwing that all away and becoming a merely average country with an average amount of smart and ambitious people. Throwing away our key advantage for literally no benefit.