Yeah, I wouldn't hold out any hope on this one, much as I would like to not have to pay VED on my 2002 Defender.
Nobody likes old cars being on the road. The Green lobby doesn't like the emissions problems, the Safety lobby doesn't like the comparatively poor primary and secondary safety systems, the Motor Industry doesn't like the idea of not selling as many new cars as they can, the average motorist doesn't like the comparative difficulty of driving them without advanced navigation and other assists, and young people can't afford to buy the petrol to put in them anyway.
Probably most significantly, politicians don't like the idea of giving up on a revenue stream, especially since the motor enthusiasts who pay it will still carry on anyway.
The argument in favour is also pretty weak. Yes, taken from a whole-life perspective keeping an older car on the road does make some sense, but it's today's tailpipe emissions that are poisoning our urban areas, and that's the current hot topic. Dropping the VED on old cars would mean having to claw the money back from somewhere else, so we'd probably just see an increase in clean air zones to further discourage their use.