You get all sorts of complaints about how older cars are "gross polluters" that are out on the highways, contributing to the Greenhouse Effect (of course, most of the "greenhouse gases" are coming from cows in the form of methane, and you don't see them being tapped...), and generally a nusisance. Especially for older cars that are poorly maintained, but are still driven because they are the only cars somebody can afford to keep running.
My solution? On a state-by-state basis (depending upon the state, we'd have to check with them to work out the best way to do it per state), the federal government will match the Kelly Blue Book price of the car that is twenty years or older when it's being traded in-as long as there is a verifiable chain of custody to show that it's going to the junkyard. Double rates if the new car that is being bought is a hybrid.
This way, you get older cars off the road-and keep them off-and put newer cars on the road. Which will have better emissions controls, better fuel mileage, and better performance. I can't see where this would be a problem...tho somebody will complain, I promise.