If you only drive 5,000 miles a year, sure, but that's not typical.
Newish used cars with ~200k on them are usually good buys because often times it's someone who used it for work (salesperson, etc.) and just ground out the highway miles (which are nearly free from a wear and tear perspective)
If we somehow manage to magically share a car, it's not going to last a lot longer than if we're wearing out our own cars at our respective rates (mine faster than yours).
But you reinforce the point that the car sitting in the parking lot or garage is not simply an over-provisioned resource by reason of being idle. If your usage patterns are so different than mine it's unrealistic to expect a configuration where we would be able to share a car.
Car-sharing has many benefits, but a car is still worth more than two half-cars.