In a way this was to be expected, whenever the government provides tax incentives to encourage desired behaviour (in this case the purchase of low-CO2 emissions cars), once they are successful they need to address the resulting shortfall in tax revenue.
It not too dissimilar to driving offences, i.e. in the unlikely event that we all drive at the legal speed limit and never park illegally - the official objectives or 'encouraged behavior' of safety cameras and traffic wardens - there would be a difficulty for the government in footing the cost of operating the cameras and wardens.
It is interesting that the new VED is based on both wealth and CO2 emissions (as opposed to CO2 emissions only at current), and that it is still not based on pollution but on again CO2 emissions (i.e. Diesels are still getting away with low VED).
I suspect that the omission (again) of the pullution element from VED is due to the fact that the government has signed-up to reducing CO2 emissions, hence national targets are based on CO2, not pollution?
But even so this does not explain the position of TfL, who base Congestion Charging in London on the government's national CO2 targets, rather than air quality in London.
You would have thought that a mayor's priority would be the immediate welfare of his/her city rather than combating global warming which is really more of a central government issue.
Perhaps this will be addressed in future by a Diesel tax in London.