Because the economy would crumble, like it or not.
--Our economy is as a house of cards? I think not.
--There are tens of millions infesting our shore already - further proof of abysmally inept border "protection".
-- A Democratic constituency would be deprived of a further voting bloc and base.
Admittedly, I'm no economist, but this statement seems to me a rather oversimplification of a possible outcome. I assume you're predicating this on the loss of workers (doing jobs that nobody else wants), and the loss of "purchasers" (that is, the people who buy "things" in our economy).
Again, I don't have a degree in economics, but in my mind, I wonder if those losses wouldn't be offset by:
A) Employers being forced to pay a decent wage for those jobs that "nobody else wants," or else be forced with watching their strawberries die on the vine and the washed cars pile up waiting for folks to dry them off.
and B) We'd have that many more illegals clogging up our school systems, prisons, and health care system, with the overall effect being lower taxes and more money in legal residents' pockets to buy those "things."
Again, these are just my own personal (and uninformed) observations.