I can't see why a PAYG system can't work nationwide.. not everyone is buying tickets that cost hundreds of pounds. However, I'd expect such a system to work with PAYG credit for people to use for short journeys on an ad hoc basis, perhaps including local buses etc - and a system to load on longer distance tickets purchased online in advance, or from a ticket window ('please touch your card here sir') after paying - like loading a Travelcard season on an Oyster.
The great thing about the national system is that if I go from London to Scotland, I may use PAYG on my local bus to the station, then load my advance ticket to Edinburgh and then step off and use the card on the local buses there, or local trains. No hassle, no exact money worries etc.
I can't see how such a system wouldn't work and if the card is lost or stolen, it's easy to cancel and replace. Joe Public will almost certainly accept them, or even demand them now they're already perfectly happy to use Chip & Pin cards and gradually getting used to contact less payments with Visa, Mastercard etc.
The quicker this happens the better, for all of us. It won't solve everything overnight, but it certainly won't make things worse.
Lots of localised systems seems the worse option, as you now need to have people sourcing another card to carry when travelling to somewhere new. And with so many people having an Oyster already, it would make sense to standardise that so Londoners already have a card they can use elsewhere (hey, it might just encourage more to venture beyond the M25!).