What of the role cheap used cars play in helping people with little money improve their lives (for instance, by allowing them to commute to better jobs) and the positive ecological effects of raising people out of poverty? An impoverished population has less leverage against corporations and empowered corporations can lobby governments to stop meaningful environmental regulation.

It's not clear to me that 'clunkers' were a net benefit to society. Maybe getting rid of them was the correct choice. I don't know what the answer is, but I think there is probably more to consider than car emissions.