if "Uber should be able to maintain their own cars at a lower cost than individual car owners can", then Uber should be buying cars today and leasing them out to their contractor drivers. Truck fleets do this, pretty much, so it's not a completely crazy idea. Uber isn't doing that, which suggests that Uber thinks its a better deal for them to have their drivers shoulder the ownership, maintenance and depreciation costs of the capital. Why does self-driving change this?

Only reasons that seems to make sense is because people think that Uber is only compensating their drivers for their labor - but they're not; they are compensating them for their labor as a driver plus the lease and use of their car.