Mainframe-style gaming is possible and they have the resources to try it so... here we are. If it doesn't take off, Google will have no problem killing the program in 3 or 4 years.
That said, I really don't see the problem with this strategy. It surely makes much more sense than having an ever growing stable of also-ran offerings. The upshot of course is that every now and then one of these "side projects" takes off into an enormously successful business unit (e.g. Android, Maps, GSuite). All of those programs could have just as easily died on the vine like Google Plus or Wave.