Exactly my thoughts. If I remember correctly, Microsoft makes much more money off Office than Windows, but they have to keep producing Windows because that helps maintain their grip on Office users. I'd wager that support costs for an OS are way higher than support costs for an app, even one as large as Office. If MS could drop their OS the company would probably be much leaner and profitable, but they can't do that or they risk losing control.
If Google was to release something, it would be smartest to release something that works on Windows, Linux and OS X. Let the support for the OS, where the biggest headaches come from, to someone else. That makes the most business sense to me.