I'm no Trumpian or Brexiteer, but:
The US has benefited immeasurably from immigration.
The USA benefited immeasurably (for a while) from slavery and also from expropriating resources by progressive colonisation/expansion and invading half of Mexico. Just because something was economically beneficial in the past is no justification in itself for perpetuating it into the future.
Plundering the skills base of the developing world (as the UK also does, particularly in the healthcare field) under the fig leaf of "But they WANT to come here!!" is at least morally questionable, particularly when it is being orchestrated as a thinly disguised people trafficking operation by corporations that are concerned with importing cheaper labour to inflate profits rather than any benefit to society or the migrants or the donor countries.