I'm not sure that's a good assumption.
Nobody is valuing taxi companies at $40b. Part of why they're so appealing from an investment perspective is that Uber's model is a heads-I-win-tails-you-lose proposition. Any time money gets made, Uber gets a big cut. Costs and most downside risks are transferred to the drivers.
That includes risks that the capital costs (that is, buying the cars) and the maintenance costs aren't really justified by the revenues. It also includes the risk that there are too many drivers available.
If I were in Uber's shoes, I would worry about the defensibility of the business model. At least here in SF, drivers often participate in multiple services. For consumers, switching is as easy as installing a new app. If Uber is consistently more expensive, then I could imagine a lot of people switching. One way to keep costs low is to screw the workers as much as possible, and pretending that employees are contractors is a pretty popular way of screwing workers.