I can see all kinds of problems with that. If the passenger that used their debit or credit card for the other passenger joining the bus, then alights at the next stop and a RPO joins later, how is that passenger going to prove they paid in the absence of the card?
OK, choose a passenger that is going at least as far as you are going. The vast majority of passengers will have a contactless card anyway before long so you are talking about a tiny, tiny minority.
There are many cities where drivers do not sell tickets, but as far as I know, they don't check tickets either. I can't think of anywhere off hand where the driver checks tickets but doesn't sell them. It is time to let the driver just get on with driving and allow ticket inspectors to catch those without a ticket. Quick boarding is top priority, above revenue protection. Before long, that will apply to 600 buses so you might as well let it apply across the network.
The end goal is probably the withdrawal of the Oyster card in favour of contactless bank cards. So no need for ticket machines in Tube stations and no need for Oyster agents. Perhaps newsagents will sell prepaid contactless bank cards by that point, which of course could be used for payment everywhere, not just for transport. Another significant advantage is that processing can be done away from the vehicle, such as using overnight batch processing, unlike smartcards which require processing on-the-fly.