And, nobody will buy the service or product from those Americans.
The price of food will go up if picked by US citizens. The cost to taxpayers will go down and tax revenues will go up. Mexico is the loser in the game, not us. Mexico will lose a huge source of income - Mexicans working abroad. It may even be enough to have the salutary effect of causing a revolution in Mexico. One can hope.
In that case, I guess we'll have to clean our own toilets, pick our own vegetables, etc.