Much of the total environmental impact of a car is in the initial manufacturing. The idea would be to increase the average age the fleet. It is plausible that this might actually have more of an environmental impact than increasing mean fuel efficiency. However, I doubt subsidy would actually defer the age vehicles are scrapped unless it is structured to encourage certain types of repairs. Implicitly this already happens somewhat with registration and property taxes taking into account market price and in some states car age, so perhaps a cross state study could be done to assess impact across states empirically.