The real question to justify Uber's valuation is "will they build something with a real moat" -- I don't think "network of drivers" is that. They did the hard work on breaking open protected taxi mafia markets, and educating the public, but it's too easy for any local city to have a dominant "Uber for our city" which isn't Uber.
Self driving is the one way I think they can differentiate themselves, and the scary thing is a lot of the departures seem to be from there. If those people have lost confidence, they're fucked.
It wouldn't be hard to turn Uber into a solid $5-10b company, but due to how financings work, that wipe out a lot of people. The only winning outcome for most employees and most investors (weighted by $ invested) is the $50-100b+ "global transportation monopoly".