My Oyster is not registered. I keep it that way for reasons of personal privacy. A lot of pre paid cards require one to register using for example ID like passport number. So are different to cash. I remember idea that Oyster could be used to buy small items. Like Octopus. It never happened. Its more likely , unfortunately, that Oyster could be phased out as contactless card payments are now possible. I don't feel governments would be keen on people having cards they could load up anonymously for larger amounts that are spent in shops and bars. Technically its probably possible. Yes it could be alternative.