I don't (as a rule) fly, so I certainly haven't visited as many far away cities as you. However, don't you find that increasingly contactless payment just works anyway?

Certainly all Oyster terminals have accepted my Android phone for some time

[If you use it constantly I believe the cheapest options still involve paying up front for an Oyster - but visiting once a week for work, or a few days on holiday the system tracks where you went, then works out after the fact what the cheapest way to do what you did would have been, and charges any contactless payment system accordingly]