A contrarian view: asset-heavy business is good for the winner because of barriers to entry. How many competitors SpaceX will ever have?
So if Uber wins the market, they'll have it for themselves.
Asset-heavy transportation businesses aren't as risky as people assume. Airlines buy they airplanes. Shipping companies buy their (massive) ships. Hertz can afford the cars they rent. FedEx can afford their truck.
Those businesses rarely go out of business and the "market fluctuation" is not really a thing. If there's a sudden 10% drop in flights, the airline will bump the prices by 10%, retire some routes and voila, they are profitable again.
I get that there's capital and risk benefit to offloading car ownership, I don't see how that translates to low margin.
Margin is whatever Uber decides it to be. If they can lower the cost of providing the service by 10x by not using humans, then they can offer a price that is only 5x lower and get obscene 50%+ margin.
What might affect a margin is lots of competition, but this seems like "winner takes most" business (currently Uber is 10x Lyft, it's closest competition in US).
Finally, getting money to fund such business is easy. Uber already raised ~$10 billion. Assuming cost of self-driving car to be ~$20k, that's half a million cars.
Given current market condition (low interest rates, lot's of money floating around), a business with a plan will have no problem raising essentially unlimited funds.