"MIG" wrote
And there's never been any proper reason for refusing to put day
travelcards on Oyster anyway. It's extra money up front, and the punter
still buys into the Oyster system.
Instead of charging an uncapped maximum single fare for unresolved
journeys, the system could charge a day travelcard instead, and let the
punter use it as such thereafter.

That sounds like a very good idea. Is there any technical reason why it
can't be done?