It's "broken" in the sense that other parts of the society depend on non-enforcement. Start enforcing it more often and the house of cards starts crumbling.
Really? If we enforced immigration laws strictly then prices for food and household services would rise a little, but the whole economy wouldn't crumble.
There would be chaos in agriculture lasting over a year, in my opinion.
Labor is only 6-7% of the total cost of produce.
Benefiting from exploitation =/= dependence, but I would imagine some businesses rely more heavily on this than others.