When you make a major change to a product that requires the user to relearn it, you lose any advantage you had over your competitors.
If you have to relearn Office you might as well consider OpenOffice, etc. too. A major change in Windows? consider a Mac or Linux as well. Vista was a great example. Make big changes to usability, for better or worse, and have people look at alternatives. Many of those flocked to the Mac and they ain't coming back. If M$ had cleaned up XP a little that wouldn't have happened.
Few people want change. Most just want to get the job done. We live in a world of forced upgrades. Your ten year old XP machine died and you want a replacement? Its Win7 or nothing. Which means your old, perfectly fine software will probably need replacement. Which means retraining. All of which adds up to extra cost that pisses customers off. Many are wising up and shopping elsewhere.
Ironically, the Mac version of Office (2011) can have the ribbon turned off and all the old buttons restored.