If you were going to make an OS, it makes sense to start with something rather than re-invent the wheel. Linux is an awesome OS but is still, after all these years, kind of rough around the edges for an end user Desktop environment. It really seems like Desktop Linux needs someone to take it that last mile. Someone needs to give it that last bit of refinement that makes the user experience truly enjoyable.
Google could do this, but why? Where's the pay off? As much as I'd love for anyone to get Desktop Linux to where OS X is today, I just don't see it happening. It would seem to make more sense for Google to develop a cross-platform thin client (firefox + web 2.0) that accesses Google Ads .. and services.