Implicit in Uber's argument is that one day it will control all, or at least the vast majority, of the ride-hailing market. That's not a want for the business; it's a need.
The positive network effect doesn't really work for Uber. Having more drivers and, as a result, more passengers, and, by extension, lower wait times, isn't enough. So long as there is competition, Uber will continue to have to lure passengers and drivers with more and more subsidies.