Earlier quoted context omitted.
Measuring how useful the car is would be perfect. GDP is like measuring how useful the car is by its market value.
Right, and usefulness (however defined - hauls cargo, go fast, fuel efficient, looks sexy) determined demand which influences market prices (assuming supply isn’t overly restricted). So market price tends to be a pretty good proxy for usefulness. People who complain about GDP as a measure are usually just complaining about things they don’t like that go into GDP like pointless (to them) consumer goods.
> So market price tends to be a pretty good proxy for usefulness.
That's quite a jump; market price can reflect usefulness, but it can reflect a lot of other things too. "This car must be useful to someone because it's expensive" is already questionable logic (at least without stretching "useful" to the point of meaninglessness), and "this car is more expensive than my current car, therefore it would be a good upgrade for me" is blatantly absurd.