Stick a video camera inside the car. If an ML algorithm detects anything too far outside of the norm, send the video to a human. If the human notices damage, the riders are banned from the system. Banned rider lists could be shared between autonomous taxi providers. End result: no one will fuck with the cars.
Except that we are humans not robots. You never picked up a friend after a night out who got sick in your car? Or had a baby that decided to smear poop all over the seats? Spilled coffee in the car? Know someone who likes to smoke in the car? That stuff happens, and if there was even a slight risk that it would get you banned from convenient transportation then yeah, people will buy their own so they don't need to worry about this. Pretty much everyone who has kids has to keep extra stuff in the car just in case - ergo, they will always go with personal vehicles. Otherwise, we'd see far more people switching to taxis than driving their own cars, especially compared to new cars, as monthly taxi cost can easily be lower than car/insurance/tax/fuel payments on a new vehicle, and yet people keep buying them.
