I read the question more asking what would happen. You said some good stuff so I'll add to it

It's irrational. Immigrants make up so much od the U.S. population. If there's 20 million of them that's ~15x more than had died from covid. (I have no idea how many immigrants there are in the U.S.) Effectively gone. Not to mention all the U.S. citizens who would follow their family members outside the U.S.

The U.S. is a farm product exporter. U.S. farmers are already in a crisis and rely on federal subsidies due to shortage of labor on top of expensive equipment and seeds.

Additionally, the communities they previously resided would be poorer. Less money spent, less friends at school (also: less funding!) Etc.

Also crappier, more expensive, rarer goods and services (less workers at restaurants, auto shops, etc.)

There's also international consequences. The U.S. has always been mostly immune to other counties whims, but that can change. Europe is a big force now. China. Etc.

It's just some bullshit to keep people mad and facists in power.