Here's an unlikely scenario: Microsoft cries "mea culpa" and admits it erred in slapping a tablet-centric interface on a PC operating system. Its penance: Starting with Windows Blue, PCs without touchscreens will run a slightly modified version of Aero, the desktop UI from Windows Vista and XP, by default. Windows 8 tablets will use Modern UI. Users of hybrid devices, such as the Microsoft Surface Pro and Lenovo Yoga, will choose one or the other. Everyone is happy. Well, everyone but Microsoft, which why this scenario is improbable.
Microsoft's mobile strategy depends on the ubiquity of the Modern UI. If you're comfortable with Live Tiles on your PC, the odds are good you'll buy a Windows-based tablet or smartphone. Making the Modern UI optional on desktop PCs essentially neuters this strategy.