Historically, Windows hasn't been tremendously effective in the area of backwards compatibility. Anyone who has migrated to a new Windows release with older peripherals has likely felt the pain I'm talking about.
On the flipside, the idea that Windows 8 will drive Windows users to Ubuntu in droves is unlikely. If a new PC buyer has been content with the Windows OS, switching suddenly to something else is highly improbably. Even if keeping their existing hardware and locating a good Linux distro might be a more economical solution, most people will stick with what they know. It's simply a matter of familiarity for most Windows users looking to upgrade.
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