>Consider a new user, with no experience in office software at all.
But is that even a reasonable share of the market any more? I'm hardly a spring chicken and I was exposed to Office in elementary school. I would imagine that the proportion of people who've never used an office suite (and can't use Google to search for answers for how to do things) is fairly small at this point.
In my experience, adding the Ribbon was a huge step backwards in terms of usability, not because the Ribbon is inherently bad, but because it was a very big, very abrupt change from the File | Edit | View ... menu system that preceded it. I was working on an IT helpdesk at the time the Ribbon rolled out (with Office 2007), and the backlash from users who couldn't apply the same set of steps that had always worked for them to do things like adjust paragraph spacing was immense.
I feel like Microsoft was blinded by its focus groups. Perhaps the focus groups included people who weren't familiar with Office, or with computers at all, and that skewed the results enough to make it seem like the Ribbon was a good idea. But in practice, the cost of adopting the UI is not just the cost of learning the new UI, but also the cost of forgetting the UI that preceded it. The Ribbon might be great if you're starting from scratch with a new UI, but the existing Office 2000 UI was so entrenched, the cost of forgetting was huge.