No one is *immigrating* into Earth. AIUI, the issue is world P, not US P.

"We" means the US.

OK, good correction.

The US is not overpopulated by any stretch of imagination.

Not at present, as far as subsistence is concerned, but then our present
population is heavily dependent on fossil fuels for our energy supply.
Replacing all of that energy with renewable sources will be difficult
and very expensive. Things may get very interesting somewhere around
the middle of the century.

We could lower the American P before then by sealing the frontiers, but that
would not only **** off our neighbors, it would crash the economy. Which is
why it's not done.