I actually don't expect they'd be successful using arbitrary people's cars.
These marketplace companies don't tend to do very well when the supply side of the market isn't properly prepared for the nature of operating in the marketplace - i.e. most Uber drivers are exclusively Uber drivers - it isn't really car sharing any more.
The time and effort would shift away from dealing with drivers who know where they stand, to providing constant support to upset car owners. Quickly, it would switch to the supply primarily being provided by large businesses who own 1k+ cars because the economics stack up well in large quantities.
At that point, Uber might as well just buy the cars directly and cut out the middleman.
These marketplace companies don't tend to do very well when the supply side of the market isn't properly prepared for the nature of operating in the marketplace - i.e. most Uber drivers are exclusively Uber drivers - it isn't really car sharing any more.
The time and effort would shift away from dealing with drivers who know where they stand, to providing constant support to upset car owners. Quickly, it would switch to the supply primarily being provided by large businesses who own 1k+ cars because the economics stack up well in large quantities.
At that point, Uber might as well just buy the cars directly and cut out the middleman.