Agree with you totally, although I'd argue that mobile is most definitely the essence of Uber's business. If it weren't for the app I wouldn't use Uber; being able to see how far away my ride is and where it is right now is the only thing that keeps me from calling a cab.
Mobile is vital to Uber's business to function, but it isn't dependent on organic traffic through the App Store to succeed. They can exist with a relatively low number of installed users because their users have an absurdly high average revenue and retention rate compared to most mobile applications.
