Uber won't capture merely the value of the US cab market, they're going to vastly expand it. People are already using Uber for things that they would never otherwise have tried to hail a cab for due to either cost or inconvenience or annoyance. It has significantly opened up the potential scale of the US taxi market, and will continue to do so. Uber's use is also blatantly going to broaden beyond the classic taxi market, whether that's to autonomous (which will again drastically increase the size of the market by bringing down the cost) and or deliveries.

You might as well pretend AirBnB is only trying to capture the hotel market's value. When in fact they're vastly expanding upon it while simultaneously taking market share away from hotels.