Sales. That's what makes a viable product. One that will sell. Despite elsewhere in this comment chain you saying "I'm talking about product viability not technical" I can't help but think that your response to me was about technical viability.
People are already willing to buy and trust their cars to drive for them right now, when they're specifically being told "don't trust it to drive for you". The second it's legal to advertise "let it drive for you!" autonomous cars will sell regardless of their ability to perform well in unregular circumstances.
But people keep talking about "self-driving car" as though it is going to be a robot capable of running errands on its own, with no human driver and not even necessarily any human passenger. The question is still open: can this be done well enough, enough of the time, solving a valuable enough class of problems, that producing such car-shaped robots will actually become a significant industry? Furthermore, will the problems solved by such hypothetical car-shaped robots be significant and widespread enough to justify the hype currently being lavished on them?
Personally I'm of the opinion that because we've been making tools for 2.5 million years and don't seem any happier than the average animal, the very concept of a new tool making all that much of a difference is absurd.
It is very clear that autopilot will be a successful feature and we can reasonably expect continued improvement in human-driver-assist technologies for years to come.
But people keep on handwaving past the very important gap between "automation which assists a human operator" and "autonomous vehicle which gets around on its own", as though doing well at the first problem necessarily makes the achievement of the second problem inevitable.
Driver assist technologies make all kinds of sense but they do not fundamentally change the nature of our transportation systems. They just make it easier to do a better job as a human driver.
Getting to the point where most wheeled vehicles are self-piloted autonomous robots with no human driver interaction...? I still think that's a really hard problem.