I think there's a qualitative difference between a business model that can sustain itself and one that relies on rich backers to dump money into it every year, forever.

Isn't HN usually full of people saying you have to figure out what customers will pay for (meaning, what customers will pay more for than it costs you to provide)? Bike-sharing is not one of those things. End-user customers will not pay you more than it costs to provide.