There is a big challenge not discussed in the article, and it could lead to significant delays for the whole business model: It's the legal and regulatory factor.
Currently, completely autonomous cars at not legal anywhere. A driver is always required.
It's one thing to have dedicated pilot programs in certain areas with an employee watching the wheel. It's a totally different thing to get nation- (or world-) wide approval for fully autonomous vehicles. (Which they need for a successful business model).
It might take years of lobbying, laws and new regulations to get that far. In my opinion Google and other newcomers to the car market (Uber) underestimate that challenge. The established players already have their lobbyists in place, and a couple of years in regulatory hell might completely erase Googles advantage.
