Road safety is a maybe, air quality is a theoretical as the numbers are so low. Contrary to popular belief, most of the cars weren't 70's gas guzzlers but late '90s models with relatively efficient fuel injected engines.

The energy used to create and deliver the new cars far exceeds the energy used to continue using the old ones.

There is no defending this insane scheme unless you want to get into partisan politics and go for the symbolism of it.