Destroying the cars was meant to raise the average overall fuel efficiency of all the cars in the US. Kind of a trickle-down plan:
1-The drive-able car itself is taken out of the pool.
2-The parts are taken out, slightly limiting the usable life of other models of that car still on the road.
3-People buying used cars are likely to choose something newer or with better fuel efficiency, as the pool of old and fuel inefficient cars were diminished by the program.
Overall, it did totally dick the population that doesn't buy brand new cars, and those businesses in the used car sector. Even people that buy 1-owner, 2-5 year old cars are still affected as the entire supply of used cars decreased, driving up the price of all used cars.