There were automated diners in the ‘40’s. But labor was so cheap they couldn’t make it. It’s not so much the direct cost of labor that makes hiring people unattractive, it’s the 45-50% adder cost and the unbounded liability that kills hiring. When you add in unemployment insurance, mandated “benefits” (socialism in the workplace dictated by government) and insurance, mandated paperwork, local taxes, etc. Then, you add on the fact that Congress has put laws on the books allowing your special protected class of workers (blacks) to sue you for imagined discrimination with the taxpayer footing their legal bill...you have the perfect storm to destroy the labor market.
All of the nation’s current economic problems and unemployment are government induced. It’s not just one thing, it’s an entire meddlesome government mentality that would have to change. But, instead of backing away they will meddle even harder.
I remember eating at the Horn & Hardart Automat on the lower level of NYC's Macy's back in the 60's.
One thing that helped kill them was inflation. They were viable when you could deposit a coin to get a plate of food. When a plate cost a dollar or more, before machines that could accept dollar bills, you had to switch to having a cashier.