The part of American agriculture which employs illegal immigrants is clearly not mechanized; they are not driving combines and are tending vegetables by hand.
Types of food really are discretionary. Beans are an excellent vegetable and can be raised completely without manual labor. Trading salad greens for them would not be an overall nutritional loss. Perhaps the true market price for a head of lettuce should be 1.6x in a properly labor regime.
To draw the distinction more clearly, suppose we had zero illegal immigrants working in the field and someone said “let’s reduce the price of some vegetables by a third and create a permanent underclass”. Does that sound reasonable?
agreed, two points toward this are
1. American agriculture is largely mechanic already, which removes the labor completely
2. food is non-discretionary, caveats about choices of what king of "food" eg. fast vs healthy