Aren't the Google Apps enough?
Next question: Why should Google create an operating system? Isn't it enough for them to create their own suite of Office applications (Google Apps) as a replacement for MS Office? Why get into the operating system battle?
Well, let's try that thought experiment. Let's assume that Google creates a suite of online software applications that are a feature-for-feature replacement for MS Office. If Google Apps provide a 100% replacement for MS Office, does this make an attack on the operating system front irrelevant?
Answer: No. The reason: Corporations don't just rely solely on MS Office, they often rely on other Windows-specific custom applications, i.e., applications written to the Windows API in VB, .NET, and even other languages like Delphi.
So, IMHO, even assuming that Google Apps is an Office-killer -- guess what? MS keeps selling Windows because there are a ton of other applications Corporate America needs that are tied to the Windows API. Google may take some percentage of users away from MS Office, but MS Windows sales will keep on trucking.
In this argument, notice that I didn't even have to discuss Consumer America to know that Windows won't go away. Now let's throw them in the mix, and guess what? Consumer America won't get off the Windows platform either because they also rely on fat client applications written to the Windows API. (Think QuickBooks, Quicken, Office, text editors, video games, music and photo organizers, tools for creating podcasts, whatever.) Unless Google is going to re-write every desirable Windows-specific application in the world as a web application, Windows is going to keep selling.
In summary, even if Google Apps is a tremendous success and takes 10% market share from MS Office, Windows will still thrive.