That’s one reason, according to Google’s just-former Android head Andy Rubin, Google acquired Motorola: a hedge against any one Android partner getting too big, too powerful.
The big question now as we come off a year in which Samsung sold 400 million phones globally is this: Is that hedge big enough?
This is something that looks inevitable, my guess is that in a year or so Samsung will fork Android so they can tie their own services to their devices, or come out with their own OS (maybe they can make Tizen run Android apps). Google made a bet with Android in order to ensure that they wouldn’t be left out of mobile, but they can’t rely on others to do it for them. The best thing Google can do is to use Motorola to produce their Nexus line of devices and ensure a pure Google experience along with updates. Otherwise, they will be at the mercy of Samsung and the others because Android is open (at least to a point) and manufacturers can do what they want with it. Today they are partners, tomorrow fierce rivals. This is a dog eat dog world.