I've seen those QR codes taped to tables in the lower end restaurants. In itself it offers an opportunity to deal with non-chiinese speakers in a minimal hassle way for local staff. It would mean offering the app to foreigners and adding some (machine) translations. .
I've paid with cash in a restaurant where it was clear I was the first to do so in quite some time. They had to think where they kept the change..
I'm not against using apps, after all that's the way the chinese seem to want to do their business. All I'm saying is that there's currently no option to participate as an occasional visitor. That combined with the fact that many merchants, in the areas I've seen, very rarely conduct cash business any more and soon it will become like Sweden where cash is rare and cashless stores become normal