Nothing about autonomous driving—how it's being pitched, the valuations, hardware, etc—suggests it's going to be cheap

Really? That appears to be one of the primary applications, and it's discussed in many investor calls re Waymo and Tesla. The driver component in the total cost of taxi service is 60-70%. EVs also have a lower total cost of ownership, and this scales with the number of miles driven because electricity is cheaper than gas. Even if the price of EVs stayed exactly as they are, autonomous taxis would cost 70-80% less than convention taxis. That's a market disrupting drop.

Investor calls have bluff to pump stock prices.

Total cost of ownership not always lower, as the depreciation is faster, insurance and repairs costs more.

Cheaper than gas? Depends on your electricity source.

In my country all taxi are LNG-powered and are cheaper/same per km as electric cars.