I agree it's hard to run a profitable restaurant, but I disagree it works fine in Singapore. I just had lunch at a QR based restaurant here and it was a bit painful. Staff shortages are a real problem though in F&B so something has to give.
Thankfully printed menus are making a comeback. Even when the apps work fine I still prefer a printed menu.
And if you don't have a smartphone, you can still just talk to the staff and order with them.
I like that this lets restaurants improve their labour productivity and lower their labour costs. It's hard to run a profitable restaurant.