
Like I think conceptually it’s a valid topic to discuss how many people a country’s social welfare system can reliably absorb. There’s valid conversations about the labor dynamics of immigrants. But SO often if you drill down on the reasoning behind social movements who want to restrict immigration it just turns out to be a bunch of racism. It means I can’t trust that people hold that position for actual normal reasons.
In the past, America was able to have fully open borders because the inaccessibility of ocean travel inherently limited how many people could come over, and there was significant frontierland to absorb immigrants. Overland immigration from Canada and Latin America was not limited because they were at similar levels of development to us, so people would go back and forth depending on labor demands.
We now live in a world where travel is easy, but inequality still exists. So everyone having fully open borders would drain the populations of developing nations and overwhelm the ability of developed ones to absorb the influx of people. The same thing goes in the other direction, people from wealthy countries would easily gentrify poorer ones (this is why American Samoa limits land ownership to Samoans).