So, we evolved out (in economic and civilization sense) of slavery, as we evolved out of unreasonable minimum wages, with the help of governments (enough people saying: no). In a complex system such as a modern western economy, one can has these cracks (work for an hour to buy a loaf of bread and milk), which is very close to slaving, and because these economies are modern in other aspects, they can buffer than. But, the road is still downhill.
The government that doesn't care about it's people, will self destruct eventually. No matter the form. It's a historical fact.
In this very narrow case of Uber, Uber will self destruct eventually, as they subsidize prices to ensure growth. Once they stop doing that, then they are just another Taxi company with an app. And most of the Taxi companies already have apps. So it will be down to the drivers (who in the case of Uber will be paid even less).
The above hypothesis will be tested fairly soon, as Uber is having their IPO this year in fact.
Uber is consensual.