Migrations and wars happen.
Demographics change. Deal with it.
That being said, if the US is very foolish, and does not protect our border, and allows unrestricting illegal immigration, and stands by while our entire Southwest becomes an extension of Mexico — then I would be the first to say “That’s how history works. That land became Mexico. Because migrations happen.”
The task for the US (as for any nation) is to protect our land from invasion so that out demographics remain controlled and we remain the USA.
“That being said, if the US is very foolish, and does not protect our border, and allows unrestricting illegal immigration”
I think that “if” has already been decided.