Wow. If everyone could copy whatever car they wanted for free, that would be a good thing. Since everyone would already have a car, car salesmen would go out of business. That's exactly what should happen.
If you physically stole cars but left the complete per-unit cost in cash there, then you wouldn't have done anything wrong, right? Alternatively, you have a machine that makes perfect copies. Both work the same for this purpose.
Because you only focus on the per-unit cost. But even cars have fixed, one-time R&D costs that are distributed on each sold car totally arbitrarily just as the fixed costs for producing digital goods area distributed totally arbitrarily on each download/CD/whatever. The only difference is that digital goods have virtually no per unit-costs to go along with it. Still, in both cases, you need frameworks to reward creative/R&D work if you want it to happen (especially in a competitive way).
If you copy all music and all cars, then everybody has a car and music, but what is the incentive to produce better cars and new music later? That is the big picture.
Yes. Cars do have fixed one-time R&D costs. If you can freely copy all cars, the incentive to design/produce a better car would then have to be something other than money. As an example, Greenpeace might want to cut vehicle emissions in half, so they'd research and develop a way to do it.
