There are good universities outside of the US. If the US stops admitting as many foreign students, they will still get just as educated. They will just get educated in foreign universities. Those universities will grow and improve to meet demand, because that is how capitalism works.
The big challenge in this idea of non-US universities absorbing the students that previously might have attended US universities is that outside the US, universities don't operate on a capitalist system with supply-demand mechanisms. The US is unique in allowing many universities on many different levels to exist. Degrees from the top 150 universities can all find jobs, and often degrees from other universities can also find jobs. Universities can expand rapidly and compete with each other. Private universities can and have been created in every state. Students can "fail" high school or college entrance exams and still find a decent school to attend, either straight out of high school or even later on in life. In states like California, there are reserved slots for community college students to transfer to the top UC schools.
This is not true in most of the world. In most Asian and European countries, there are a limited number of universities, and there is a single exam that gates entrance to those universities. In a very real sense, these universities pride themselves on the scarcity of their enrollment slots. They will fight to maintain that exclusivity. There is no chance that those countries will expand enrollment in their decent universities. Even in China where the number of students is huge, the urge to expand enrollment at the decent universities is strongly resisted, and China is not unique in that regard.