If the government ceases funding bas passe, the bus companies will introduce their own schemes. There is no subsidy for older people on trains, but there is still a Senior card available that gives reduced fares and other benefits.
I think anything pensioners get that is other than cash in the pocket, free prescriptions, the lot, should go, it is demeaning, suggests that we are too old and stupid to manage our own money and set aside money for buses or nedications. In their place I would put up Pension Credit sufficiently to cover the extra costs. This would also give help by bringing into Pension credit, a significant number of those just above it, so that they do not suffer.
As for bringing people on the roads who weren't there previously. I doubt many people would go out and buy a car because their bus pass stopped - but all but the better off would see their pension increase to cover the extra cost, and many who already own cars would probably still prefer to catch the bus, to save on car park costs and difficulties.