Why not just use Oyster or an ITSO based product instead of another pointless expense? It's bad enough the amount of GLA taxpayers money that was spent in acquiring the rights to Oyster.
If Oyster continues indefinitely then there would be little financial advantage to TfL/TOCs in having contactless cards as an additional payment method, except to enable the withdrawal of cash from buses. There's a case for saying that ITSO/Oyster is just for season tickets and contactless payment would be for pay as you go travel nationwide.
The main problem here is the inability for some people to access basic banking facilities, which should be remedied as a matter of urgency.