But driverless taxis aren't a commons: they're the opposite, a private resource managed by a for-profit company. I don't see how you're predicting a tragedy of the commons here. If you're a difficult client, then you get fined heavily and kicked off the service.
(And your fears are precisely why you'd expect corporations to run things like this: they'd respond to other customers' fears of "dirty, vandalized" taxis, and do what they can to avoid that image! You don't get that responsiveness from a commons).
The commons example as it relates to vehicles themselves is not optimal because you bring up a good point - in an optimal market situation, competitive forces will drive poor performers to do better or fail.
I’m not actually totally sure what point I was trying to make, or if this will ever become a problem, just food for thought/discussion.
(And your fears are precisely why you'd expect corporations to run things like this: they'd respond to other customers' fears of "dirty, vandalized" taxis, and do what they can to avoid that image! You don't get that responsiveness from a commons).