Cars have the benefit of transporting humans and goods around.

It's more like saying a hypothetical car which moves itself by using gasoline as a propellant rather than fuel for its combustion engine would have negative value.

Sure, using fuel (of all things) for propulsion would be one way to move a vehicle, but it would be inefficient by design.

Bitcoin, at least, was created during a time where there was no alternative to security-by-inefficency, but PoS and other consensus mechanisms are pretty battle-tested now

But cars are useful.

Diminishing value certainly it's called depreciation