There has been much criticism of the fact that the abolition of paper One Day Travelcards, and of paper tickets generally, will effectively prevent Londoners who don't have a bank account or use the internet and who aren't old enough to have an Oyster 60+ card or Freedom Pass (or have a disability that qualifies them for the latter) from using public transport.
However, whilst that is a perfectly valid argument against the proposal, I suppose in theory there's nothing to stop such a person from buying an Oyster card from a London Underground ticket vending machine using cash and then topping it up at a LU or National Rail TVM (or at an Oyster Stop shop, or whatever they call them these days) using cash each time they need to top it up. Still more of a faff than simply buying a paper ticket, though, and probably beyond the wit of some if not many people in that kind of situation, especially people with learning difficulties.