I haven't looked at the Budget so I didn't know about this, but it's a bit of a shocker.
Stinging people who own 20-year-old cars isn't going to help the government's ledger very much, but it's really going to hurt individuals who - quite reasonably - are running a perfectly functional car with years left in it.
Assuming that car lasts for another 10 years, it's still going to produce less emissions than the production of a new car, (unless the manufacturers have managed to circumvent the 2nd law of thermodynamics).