That is simply not true.
In fact, the major impetus for creating the Ribbon was the Office team dealing with the constant feature requests for features they'd already implemented, but users couldn't find how to activate/use.
> and reduced the number of features available.
Also untrue, unless you're going to say things like "being able to drag a menu off the menu bar and hover around like a toolbar" are a "feature".
Or put another way, it did reduce configurability, but the number of actual useful features was not reduced.
> Is it just a discoverability problem, or did Microsoft go with that stupid new trend of dumbing down software to the lowest common denominator?
You should know I mentally translate the phrase "dumb down" to, "I don't like this because of a knee-jerk but I've never bothered to think about it long enough to express why".
> The less you demand from your users, the dumber they get.
Seriously?
How about: the fewer obstacles you put in front of your users, the more useful work they can get done.
I think that the ribbon design itself actually helped discoverability, at the expense of familiarity for long-term users of the old interface (OTOH, I think that simultaneously with the introduction of the ribbon, they also made it so a lot of options that previously were on the UI required customization to actually have them surfaced at all -- which they also, IIRC, did in some previous versions without changing the UI structure -- and, IIRC again, they reversed some of the most egregious of those in 2010.)
You brought up Office's ribbon. Sure, it was a "look&feel benefit", but it also significantly hurt discoverability and reduced the number of features available. Nowadays I can't find the more advanced stuff I know that worked in Excel and Word in the past. Is it just a discoverability problem, or did Microsoft go with that stupid new trend of dumbing down software to the lowest common denominator? I don't know, but the interface doesn't allow me to find out.
People bitched at Chrome because removing "http" was seen as a first step of removing the address bar altogether, dumbing down both the browser and peoples' perception of the Internet. The less you demand from your users, the dumber they get. Sure, it may be good for the quarterly sales report, but it's disastrous for the future of technology.