I assure you that you do not know for a fact that H1-Bs have prevented the US citizens from getting postdocs. Partly because postdocs are hired on J-1, not H1-B.
In any case, postdoc salaries come from research funding, which is basically a fixed pot for the entire country. Salaries for American postdocs would not rise if the labor supply falls. We would just have fewer postdocs and less highly skilled labor.
At the PhD level, if we admitted every American who applies without evaluating their record, we would still need to admit foreign students. Few people want to sign up for that challenge, and it just so happens 95% of all other people are not American citizens.
For tenure track faculty, we hire for fit and quality without regard for nationality. This is a requirement of Federal law, and it makes the country better overall.
Qualified foreigners have no advantage over qualified Americans for academic positions. If you don’t believe me, most state universities have salaries publicly listed for their faculty. You could easily see if foreigners are paid a lower wage.
An argument against foreign students and academics is an argument for a smaller scientific workforce, not a more “American” one.
You can wave around theories, or we can discuss what actually happened with the program. In the 1940s and 50s we had refugees, but we also had an extremely robust academy producing the likes of Feynman, Wheeler, Thorn, Everett and many more. Then we blew that up, because foreign students were cheap virtually indentured servants.
Enter today's world where tech is doing the same. America doesn't have an obligation to favor or subsidize foreign employment. If we had a system producing more Wheelers, Feynmans, and Thorns, it would still be worthwhile to immigrate and work with them, for a full salary. The same goes for tech. I want to attract foreign students and foreign workers to the American system, not to the American subsidy.