True, the money involved is one way to break it down.
Another way to break it down though is the the amount of exhaust emissions emitted by each clunker compared to the emissions output of each vehicle being purchased as a replacement.
Bottom line (pun intended ), if the environment got so screwed up in the long run that it was no longer liveable, there would be nowhere to spend the money being saved on the fuel being pumped through those vehicles that were continuing to exponentially screw up the environment.