Given that most stores use electronic/computerized tills, ringing the purchase up is more of an issue than receiving payment. My phone works during most power outages, and unless the internet to the city has gone down, you should still be able to make payment.
I have seen plenty of 7-11s refuse cash during a power outage, online banking is not the issue here. recording the purchase is.
And how does a quick scan and transfer take longer than using a credit/debt card? Last time I tried to use my card they tried 2 machines and asked if I could scan, but i had forgotten my phone so i had to come back and try again later.