London already doesn't allow cash on buses. There's huge costs in installing bill collectors. People paying cash on a bus slows everyone else down. Making everything electronic would solve a lot of issues.
*Anyone with a bank account. Or is this a less significant problem in the UK compared to the US?
In Sweden (in Skåne at least, since the rail/bus solutions are all regional there) you have both options. You can buy your tickets on an app that you scan on the bus or train, or you can buy a smart card anonymously with cash when there is an open newsstand/rail station and use that to beep onto trains and buses. So if you go the anonymous route when you need to top up your smart card with money you can do it with cash from a kiosk in an automated machine 24/7, so long as there is a kiosk around (every rail station has them).
With a fair bit of planning it’s not too hard to stay completely credit-card and app free with this solution. You just have to make sure that you are near an open rail station/newsstand to purchase the smartcard initially (and I think even some of the grocery stores sell them) and then onwards have enough money on the smart card to get you to where you can top up from a kiosk. Given the rail coverage in Sweden it’s not too hard to do.
The ideal system is one in which you use the "universal transit payment card" desired in the article to get on the transit system, then use it again to get off (if the transit system requires it - no all do), without having to give even a single thought to how much money you are spending.
When you have different payment cards/passes/tickets in different systems, you have to figure out not only how to buy those tickets, but how much to buy (so that you don't depart the area having vastly overbought - or so that you don't run out of loaded fare value before you end your trip).