Maybe the party with biggest publicity budget wins, or maybe it's the one with lower overhead (in things like publicity), or maybe it's the one that pays better the drivers...
Or maybe there comes a company with a really open network, and destroys the entire market in exchange for a couple million (what is a good thing, by the way).
Engineering for an app like this is non-trivial but hardly insurmountable.
The one advantage they do have is network effect, the more drivers and users they have the quicker it is to get a ride and more lucrative for drivers. That does matter, the question is how much.
Someone like Hertz or a General Motors already have fleets and know how to manage them. All they have to do is engineer an app.
What is easier? A nationwide vehicle fleet or an app that connects drivers to riders.
Imagine a new startup launches today offering ridesharing in NYC. Drivers - who already have Lyft running - add the app to their roster. After some advertising spend, so do users. What is Uber's strategy to stop them? Spend more money, usually - more advertising! More drivers! Is that sustainable?
What is Uber's moat?