The big problem here is you've no fall-back for when the place you're trying to pay, or their merchant services provider, has a communications/network outage or malware attack or has assets frozen - some of which affect multiple large retailers.

There have been many instances over the years, even just in the U.K., where small and large stores have been unable to accept digital/network-required payments and ended up with massive queues or customers abandoning shopping carts full of items due to not having cash.

"Spar cyber attack hits more than 300 convenience stores" (2021-12) https://www.bbc.com/news/uk-england-lancashire-59554433

"Manx Telecom broadband outage affects 4,000" (2020-08) https://www.bbc.com/news/world-europe-isle-of-man-53920785

"Wirecard: 'It’s really bad. I’m left with nothing'" (2020-06) https://www.bbc.com/news/business-53222181

"Debit card glitch means thousands charged twice" (2018-09) https://www.bbc.com/news/uk-england-45400234

"Visa (Europe) says service returning to normal" (2018-06) https://www.bbc.com/news/business-44335804

"Asda card machine fault leads to queues at checkouts" (2016-10) https://www.bbc.com/news/uk-37815973

"RBS says it has resolved debit card computer glitch" (2016-10) https://www.bbc.com/news/business-37658559

"Lloyds Banking Group says card problems sorted" (2014-01) https://www.bbc.com/news/business-25907000

"Card payments system crash disrupts shoppers" (2010-10) https://www.bbc.com/news/uk-11458962

Also, interesting and somewhat unexpected contactless from long distance!

"Contactless 'charging errors' at Marks and Spencer" (2013-05) https://www.bbc.com/news/business-22545804