Are Hybrids Good for the Environment?

In a post of the same name a writer asks whether hybrids are good for the environment.

More specifically, the writer wonders whether purchasing a new, highly fuel efficient car (such as a hybrid) has any benefit over purchasing a used, moderately fuel efficient car. The writer's argument is that the production cost of a new car (in ecological terms) is so large that it reduce any ecologic benefit gained by the reduced fuel economy.

It's a valid question, but the solution is probably not as simple as the writer suggests. There are a few reasonable objections or follow-up questions to the initial proposition, but what gets me thinking is this: if all ecologically conscious consumers purchase used cars, are they really hurting themselves?

Let's not omit an obvious fact: the used car market is predicted by the new car market. This is so obvious it sounds dumb, but if the ecologically conscious consumers consume the larger portion of used cars, it will be the ecologically unconscious who consume the larger portion of new cars.

Carmarkers, who respond primarily to market demand, will produce more new cars that are likewise ecologically unconscious (since nobody wanted the fuel efficient cars) and the ecologically conscious consumers will find that when their fuel friendly used cars expire, all cars, both used and new, will be less fuel friendly than ever before.

Or so it seems to me.