Unfortunately it appears to be turning into good old protectionism and closed shop unionism by the back door.
The public will be forced to pay for new cars.
I also can't see much environmental benefit with respect to Co2 output.
Take an XM 2.1 diesel and a current 2.2 diesel.
The fuel economy isn't very different due to increased vehicle weights and reduced engine efficiency, due to lowering of compression ratios to reduce Nox output and smooth rough running of direct injection engines.
IMO we should be bothering about fuel efficiency/Co2 output, but as long as vehicles can achieve sensible limits then leave them to run.
In reality most older vehicles do much reduced mileage anyway so the proposal isn't well thought out.
If you need to cover 20k per year, just buy 14 Xm's...or don't fix the milometer when it goes dodgy