If the reality of the situation is that consumer devices all have touchscreens because of overzealous OEMs, then affordances should be made for touchscreen interfaces wherever possible. That said, if it's not clear that touch makes sense as the primary input device, more so than the mouse, then the UI should not be designed around it, regardless how many people own a touchscreen that they keep two feet away at eye level.
If, on the other hand, you have a device where touch is the only input that makes sense, then your shell needs to be designed around it. Every aspect of user interaction needs to be reconsidered, because you can't expect your users to run regedit with a touchscreen.
I think you're right that Microsoft won't drop Metro now, but I don't think it's a given that customers will ever adopt it. Windows 7 works fine for everything I use it for, and without architecture-imposed RAM limits, I honestly can't see a time within the next decade that it will stop doing that. I still know people who use Windows XP, 11 years after it was released. The fact is, they all run Firefox and Chrome equally well. Your desktop OS has never mattered less than it does right now.
Three years ago I bought a Macbook Pro, ostensibly for iPhone development. I went in with the assumption that I would still primarily use my Windows desktop, what with the power and the keyboard and the mouse. Within thirty minutes I was signed into my GMail account, browsing my RSS feed, and realized that I had completely migrated my life over to the new machine. In this environment, where switching to Ubuntu requires less of a lifestyle change than Windows 8, why would anyone ever upgrade Windows?