This is not so insightful.
1- Foreigners who do come to America and then leave after a short period (a few years) do not take long-term jobs away from Americans. Clearly the jobs these undertake are like internships, post docs and other temp positions, these jobs are not meant as career jobs who would be of interest to an American.
2- Foreigners who come to America, get some training and then leave are *good* for America. These people will know and like America, will speak english, will have a network of friends and people they know back in America. If they start companies, maybe these companies will be friendly to America as well: import stuff from there, rely on American technology, and whatnot. The importance of creating goodwill cannot be overestimated.
How people who come on a H1B for a non-training job, and then stay by being sponsored for a green card, this is a different story. But notice that these people eventually become American. This has been a recognised way to extend the power and importance of the USA for a long time, because the best and brightest come to America to the detriment of the country they leave.
In reality the job situation in the USA is not nearly as dire as some people make it, compared with most other countries around the world. What is not so nice is that unemployed people have it very tough, very quickly. Better not fall sick.