It's a rough estimate rather giving an idea of the market. What I want to say is that the market is not the $10B U.S. taxi market, but that Uber's market is probably in the trillions over the next 10 years.
Or $0. If you're talking about self-driving cars, you also need to consider that more than one family may share one car without any need for Uber, or that car manufacturers or dealers may go into taxi business themselves.