Tons of immigration is great given your country is good at:
2) assimilation and cultural confidence
If your country is bad at capitalism and has a welfare state, then it works better to limit immigration to people who have money or valuable job skills (e.g. fully trained doctors), not people who will go on welfare.
If your country is bad at assimilation, it works better to limit immigration to people who already share similar values. For the US, that means mostly immigration from the English speaking nations.
The US needs to limit immigration – like Trump proposes – because we're bad at both these points. Yes I'm a classical liberal who would like free movement of persons as a matter of principle similar to free trade. But we don't live in a classical liberal society. Many classical liberal policies don't work when individually used in a culturally-relativist welfare society. Bringing in a bunch of anti-liberal immigrants who are going to stay anti-liberal is not a classical liberal policy!
Taking tax money from some Americans to give handouts to other Americans is bad enough. Bringing in a bunch of immigrants, then raising taxes (or debt) to give them handouts too, is stupid and destructive. Immigration should be for people who will support themselves.
Having millions of citizens who dislike our country is bad enough. Bringing in millions more people who don't like our country is stupid and destructive. Immigration should be for people who like American values and, while not necessarily understanding our values very well already, are willing to learn and change. (For example, someone who doesn't know English, and doesn't want to learn it, should not immigrate to the US.)