Yes, fair point. If Tesla has a monopoly in the autonomous driving space, they would still be covered. Tesla's small fee in their driverless cars would still be far more appealing than a P2P system which used all other non-autonomous cars.
But if there comes a time where there are 4 or 5 companies with FSD capabilities, Tesla is back at square one because a P2P system can still exist without Tesla's technology. Liability would be given to the other successful autonomous driving companies.
For the record, I do believe Tesla will have some kind of monopoly power in autonomous tech, but this blockchain argument really comes to life if Tesla doesn't.
One other thing to consider is that by the time there are 4-5 companies with mature FSD technology competing in the ride sharing space, would individual car ownership still be the primary source of the ride sharing cars? From a user perspective, if the service is so ubiquitous and one can get a car whenever needed, why own a car and worry about parking/charging? From the car maker perspective, why sell the car and let user collect some of the ride sharing profit, with the car maker still responsible for liability, when you can cut the user out completely and keep all the profit for the whole life of the car?
I can envision, at least in urban setting where population density is high, and space is cramped for each individual to park/charge a car, Tesla can build a parking garage a few miles away from downtown where tons of apartment and office buildings are, any time a user needs a car it can be available in under 5 minutes. The cost of this parking/charging infrastructure would be much lower than individual ownership, both due to the freedom to choose a less expensive location, and also due to economy of scale where Tesla can deploy autonomous tech such as charging robot snakes at a centralized location. For example, there could be robots that can access the interior to cleanup and puke on the seats/floors, not much different from the assembly line robots that can enter via the glass roof for assembly.
In addition to just an Uber-like service, the car maker can also offer a stable lease service. User can sign on for a monthly fee, which covers certain pre-arranged rides, such as morning/evening commute, so a car shows up at your place each work day without you having to call it, so you don't even need to wait at all. You can choose to pay more for a ride all by yourself, or pay less to share with other people that live/work in same places as you.
The possibilities are pretty intriguing to me. Blockchain could be integrated as a part of the infrastructure, not necessarily a competition.