I don't see that at all. It's not that humans are replaceable as much as the fact that each driver's performance (and corresponding rating) is crucial to their continued employment.
I have no problem with that at all given the fact that Uber's image and product both rely on having good customer experiences.
If Uber were more concerned with providing a living to its employees, then they likely wouldn't last very long as a profitable business. $21/hour is a fair wage (a living wage in most locations, in fact), and that's all they really need to provide to be good corporate citizens.
Not to mention the "people are interchangeable" concept actually dates back to at least the Industrial Revolution, and is not only not limited to startups, but is probably less prevalent in startups than in manufacturing or corporate jobs in general. So I'm not sure what you're complaining about.