With self driving cars, I believe Google is showing willingness to develop things that could severely injure someone.
disclosure: I work for Google. I have not been involved in toothbrush test discussions and do not work on robotics nor cars.
If anything, the Roomba proved that there is small niche-market demand for robotic automation in the home, and economies of scale for goods follows the demand. Consumers simply aren't going to spend the equivalent of buying a new car for something that can be done by a minimum-wage maid service.
a leased new car costs the consumer a few hundred dollars a month, which is the equivalent of say 30 or 40 hours of minimum wage labor. a robot that cleans floors, prepares food, washes clothes, folds laundry, cleans windows, serves meals, puts dishes away, feeds the dog, waters plants etc. could work 24/7 (with swappable batteries). i'm not convinced that consumers wouldn't pay that if the technology could get there.
of course we are far from a robot that can do all of that, but in my opinion, the prize to the company that can deliver this is massive.
It's an inferior service, the same reason the Roomba didn't put Dyson or Hoover out of business.
Also, how big would the market be if you could just say pick up that widget and put it over there on that belt. And, it said no problem boss.
And by the way it worked with any slim margin robot arm.