Beyond using 10k/yr as a rule of thumb for average duty cycle mileage is a horrible indicator of vehicle wear. Engines are worn by cold start cycles, transmissions are worn by shifting, brakes get worn by stopping and suspension components get worn by potholes, etc.
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)
Except I never made the claim that nominal mileage objectively indicates vehicle wear. All I'm claiming, is that mileage is the largest factor. It doesn't matter if your driving wears out a car in 300,000 miles and my driving burns out a car at 80,000 miles. Your miles are your miles and my miles are my miles.
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.
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)