[agumonkey]

Some people linked the notion of user evolution pace. Evolving a system faster than their user is asking for troubles. Even if the ribbon was a beautiful idea, well executed by Microsoft; it imposed a massive relearning phase.

[Cyph0n]

I guess they had to make such a huge change eventually. I'm sure they weighed the benefits of improved UX for new users against the learning required for existing users. In any case, they had (and still have) a near-complete monopoly on office productivity software, so they wouldn't lose much either way. Worst case scenario is going back to the older UI if the backlash was too much.

[glhaynes]

People seem to have kept up.

[TeMPOraL]

Thus the rule oft forgotten in the UX circles - if your user has no alternative to switch to, they will learn the interface.

[__jal]

...Which explains every enterprise software UI ever made.