Uber, and I imagine Lyft, are banking on automation. So what is sustainability? Self driving vehicles, at scale, could profitably provide service for a lower cost per mile than a person who owns their own vehicle. And that industry is going to have incredibly rapid growth likely culminating in a monopoly that could yield the largest company in existence. We drive in the ballpark of ~300billion miles per month [1]. Consider the worldwide market. A profit of even just a few cents a mile starts to look very insane very fast.

[1] - https://www.fhwa.dot.gov/policyinformation/travel_monitoring...