"Being forced by their employers to work on Windows XP has been driving people toward Macs and tablets," Johnson said. "Microsoft's future viability depends on how fast it can get new value in the hands of users. A high cost of upgrading has been slowing them down."

However, the upgrade incentives can be a double-edged sword, because touchscreens, touchpads and mice that support gestures are important to providing a good experience with Windows 8, according to Michael Silver, a Gartner analyst.

"Most PCs that will really make Windows 8 work well won't ship until the OS does. People upgrading older PCs is actually a risk for Microsoft because they may not be as happy as users that have new PCs tuned for Windows 8," Silver said. "Users that upgrade should ensure they have either a touchscreen or a touchpad or touch mouse that supports gestures in Windows 8."

Microsoft should help users understand how suitable their existing system is for Windows 8, but it's unlikely the company will go as far as to advise users to reconsider their input devices during a Windows 8 compatibility check, he said via email.

"People who don't have the right devices for working with Metro will be wondering why they [upgraded], at best, or regretting the decision, at worst, and any bad buzz on this is bad for Microsoft," Silver said.