> The US has in the past put restrictions on who can immigrate, and continues to have restrictions (whatever moral judgement is attached to them is left for the reader). You can restrict the countries they come from. You can restrict the education levels. You can restrict the number entering. You can restrict how long they need to be here before they can vote. And so on.
That won't help, and possibly makes things worse. For example, I'm part of the high-education, high-income Asian immigrant influx to the U.S. over the last few decades. Our immigration may present a temporary financial boon in industries like medicine and tech. But, as should be obvious, there's cultural risks to importing another society's elites. We bring our elitist attitudes and affinity for top-down administration of society with us. Many Americans would, quite reasonably, say those aren't good things.
I'm not saying we shouldn't have immigration. But more than a quarter of Americans today are immigrants or first-generation children of immigrants. That's a tremendous burden on the country's ability to socialize people into the country's values and principles. And it's a tremendous source of social distress and conflict.
> Moreover, maintaining the current US culture is not entirely desirable. It has its good points and its bad points, but in general, a lack of change tends to become a regression rather than improvement.
Culture changes without foreign influence, though.