Most of the plastics today are recyclable. Most of the plastics used by auto makers are made from recycled plastic or recyclable plastic. If they don't use plastic, they use cheap, flimsy sheet metal or some alloy. Any steel alloy is not lightweight. It's still steel. They just mix it with copper, nickel, tin, or ather metals to make it stronger or more flexible.
Today's use of metal is useless. Plastic would provide the same protection that the sheetmetal provides while reducing the weight. This actually does not increase the amount of petroleum used since they are made from recycled plastics.
True, andy77's 20+yr old car is made of stronger metals. But 99% of the cars out there today that use metal siding offer no more protection than the ones with plastic or fiberglass siding. In fact, composites and fiberglass are lighter than steel siding and stronger. Some can crack or become faded and brittle over age though. So there pros and cons to both.
Even if a car is made with new plastic, the car is still lighter. If the car only improves by 1mpg, over the life of the car much less petroleum is used for fuel.
The cage and frame of the cars could be made out of some foam-infused aluminum compound or other composite that is actually stronger than steel. But since steel is more readily available and cheaper than those kind of composites, I don't see that happening anytime soon.
It's kindof a mind-game though. A car's frame isn't that expensive to produce, even with a high-strength, lightweight composite. The auto manufaturers are still selling cars in upwards of $25k in the masses. So if that kind of construction were offered for around the same pricetag, why wouldn't people buy it? It would be lighter, getting better mileage. It would be stronger, less likely to break or warp. But who am I to say that? I'm just a nuclear engineer who's seen these materials in practice, not an automaker CEO.