Supermarkets I go to have very simple two or three line LCD price displays.
They couldn't display a QR code.
So you'd have a significant investment cost to replace them with terminals that could. And, of course, of developing, testing and validating the software.
You'd also have the issue of ensuring that you had sufficient phone network coverage at the tills for all major networks, otherwise people wouldn't be able to pay.
You might save on per-transaction costs, but there would be significant up-front costs.
At the moment, would there be enough Bitcoin transactions to recoup those costs?
Plus, are people currently paying in cash likely to be those who would switch to paying digitally? Isn't it more likely to be those customers who are already paying with debit or credit cards?