But this does contradict your point that "nobody installs apps from outside of Google Play". And, if they're installing something like a loyalty app, they won't

But this does contradict your point that "nobody installs apps from outside of Google Play". And, if they're installing something like a loyalty app, they won't care very much whether that QR code leads to Google Play or to an apk. The process is really no different than running an .exe you downloaded on your computer.