>but the population of the United States is 97.9% not from this continent.
This would be a surprise to the 85% of us who were actually born here. In its most simplified form, what those of us who are skeptical of the current immigration regime are wondering out loud is if these processes do actually make "people born in America" better off. "Immigration is always good" has been the mantra since, as you speculate, the 1960s. Probably worth evaluating that idea from first principles from time to time.
It is worth noting, though, that after the peak of immigration in the late 19th century and ending in the 1910s, the United States very intentionally shut off immigration to allow time for assimilation to do its work.
There is a so-called "Technology Alert List" — a list of critical areas (nuclear, missiles, AI, etc.). If a person has a background in one of these areas, they get an automatic U.S. visa refusal during the interview.
Their visa application gets placed on indefinite hold (the so-called "administrative processing"), which can last for years even if it is eventually resolved.
Why? The U.S. government fears espionage. They worry that someone with expertise in a critical field might immigrate to the U.S., secure a job at a company with access to sensitive, export-controlled technology, and then leak that technology to Russia.
Even without espionage, such individuals could gain valuable experience in critical areas and later emigrate back to Russia — a reverse "brain drain."