> The number of drivers needed at any given time is completely variable. It's obviously a larger number during peak travel periods.
That's variable based upon time of day, not completely variable. It is not a new problem either. Take public transit: drivers may hate split shifts, yet they are given well defined shifts that they are paid for.
On top of that, a company that operates through an app ought to have the ability to develop software to relatively reliably predict demand.
> Moreover, there are many people willing to do that.
Willing, or desperate? For example: relatively few people want to be on-call replacement workers. They either do it because they need the money or they do it because they are hoping to get their foot in the door. Those who do it willingly are typically doing so for extra cash and because they don't have any other obligations (e.g. retirees in some fields). Now imagine that an entire company is based upon the concept of replacement workers. That puts Uber closer to the exploitive end of the spectrum than the opportunity end.