What if's can be used to kill a conversation or discussion because they are impossible to argue. "If there's no Electricity i'll just use my diesel generator to power the store. WhAt if YoU ForgoTT to FueL IT?!". There is no discussion to be had because one side is not interested in it.
What you should be asking is What is plausible. IS it plausible that you buy a phone that doesn't support the app? No, there's two OS's for mobile phones. They will have an app you can pay with.
IS it plausible you forgot your phone or it got stolen? Yes. But you are just as likely to have your wallet stolen. So its not really applicable.
"How are you allowed to function if the app is missing?" Is an illogical question. The apps already exists in several countries and works flawlessly.