A full legislative history of immigration is too much for this blog post, but roughly speaking there were several decades in the middle of the century during which all legal immigration was sharply curtailed, and then it opened up in various ways. I don't want to be like "imagine a country without taco trucks lol!!!" but more broadly it's hard for me to imagine this country with the cultural stagnation and homogeneity that would have been the consequences of keeping the borders closed tightly. Yurp's changed a lot in the last 15+ years or so, but one of my dislikes of it (Yurp is not one country, of course) used to be that there was something suffocating about the lack of cultural diversity. Stagnant places become self-parodies, their own boring museums to themselves.
The Trumpkins want to stop all immigration by brown people and make life hell for the brown people currently living here, whatever their legal status. The racism is sick, but it's also a horrible way forward for the country otherwise. Immigration has kept this country alive, and I shudder to think about what some of the trend lines would have looked like without it.