The theory is that the buyers of labor in these cases will simply buy alternatives - specifically low-labor alternatives, like automation. Think of automated checkout stands at retail stores, but applied to fast food.

Time was, fuel filling stations all had attendants. Drivers didn't get out of their cars when they purchased gas. They drove up and were greeted by an attendant who operated the pump and took payment. We don't do that anymore - now that attendant job doesn't exist. There's nothing stopping that same kind of transition for many other service jobs.