There's obviously the jobs immigrants do (and it's not JUST agricultural work—so many industries would be affected), but there's also the money they spend.
Immigrants buy things here. Everyone needs groceries. And clothes. And everything else. You think we could suddenly disappear 20 million people and there would be no economic impact?
Really? Really?
"Mass Deportation" would be our Brexit (but a million times crueler). It would be a disaster.