Sure, Apple, if not the biggest company in the world, then way up there, with tons of people just waiting for their next product is circling the drain. Right. And the "Metro" gimmick is not useless on a smartphone or tablet, and I don't know if you noticed that one of the "Start" page tiles takes you to your desktop for a desktop system (not sure why they don't have a "desktop" mode so that in that mode the system opens with the desktop, not my decision). The "Metro" design also has a certain artistic simplicity that appeals to a lot of people aesthetically, as well as providing exactly what you say that Google could provide someday...a single ecosystem for smartphones, tablets, desktops, and even servers if they want to, although MS has separate server OS's. Windows is already in the Enterprise due to their range of software products, Apple wishes they were, but doesn't need to be.
If you think that Google is going to put Apple and Microsoft out of business, I can't say that I see it that way. I personally don't like Android, never have. I put up with Chrome because it is the most secure browser as part of a larger system, not because of it's wonderful usability. You're going to have a hard time convincing people that Google has all the answers, they really have very few clues about designing a user interface or a product that is attractive on the basis of usability. Functional yes, usable, not so much.
There's a place for all 3 of these companies, and you're just going to have to get used to that *smile*.