BBC's Sunday Politics programme requested a Mayoral briefing from the end of 2012, which says:
It is also proposed that the sale of cash tickets on bus [sic] be stopped at a suitable time in 2013.
1.5% of bus fares are paid for in cash, 24m journeys a year, equating to around £57m in revenue. On the Sunday Politics Leon Daniels, TfL's Managing Director, Surface Transport, said it costs around £10m-£15m to collect cash fares on buses (though the BBC's Transport correspondent, Tom Edwards, said on Twitter yesterday that the figure is more like £35m) and TfL's plan seems to be that once revenue is less than adminstration costs, cash fares will go.
However, since cash fares are £1 more expensive than Oyster it's probable that anyone paying in cash has a genuine need to: lost Oyster card, empty Oyster card, late at night, etc. Labour London Assembly member Val Shawcross said:
Being able to pay with cash on buses must continue as an emergency option for people who have lost their Oyster cards and need to get home. It’s not safe to take this away from Londoners who might have no other choice and could find themselves stranded.
London Travelwatch also makes the point that even though paying with cash can slow things down, that's unlikely to be cancelled out by arguments with the driver over not being able to pay in cash.
Leon Daniels reassured Sunday Politics viewers that:
No vulnerable people are left behind on our bus network. [Bus drivers] make these decisions every day.
(Embarrassingly, he also made a bit of a cock-up, saying that if people buy a rail ticket to 'London terminals' or zone 1 they get unlimited travel on the bus and underground network. He seems to be confusing standard tickets with a ticket-plus-travelcard.)
The contactless card system isn't yet a replacement for cash, either. One million journeys have been paid for this way since the system was introduced in December 2012, but that's peanuts compared with 24 million annual cash journeys. Contactless payments will inevitably rise with familiarity, but it's also not a solution for poorer people who can't have debit cards. TfL says it will consult before any changes are brought in, but what's your opinion?