"What's stopping an upstart company to buy/lease 10,000 self-driving cars and let them loose on the street?"

You're right that self-driving cars would be a major challenge/opportunity. Re: what's stopping an upstart, letting self-driving cars loose on the street is inefficient. There is a huge amount learned/accrued intelligence involved in how those self driving cars are orchestrated for optimize the marketplace. So that's where a key capability is going to be important even in a world where self-driving is commoditized.