The Menus were a toolbar before the ribbon existed, which is why you could customize them. Visual Studio was the same. So it was natural for the ribbon to eat up the menus as well. Certainly that's part of why people disliked it, but the artificial separation between 'menu' and 'toolbar' was already kind of a problem and that's why they were unified.
People disliked Ribbon because it took a logical system geared towards power users (hierarchical menus) and replaced it with an organic system geared towards new users (usage-driven ordering).
There was no way that wasn't going to piss off a very vocal segment of users, and Microsoft, to their detriment, badly botched the messaging.
My point is that even if Ribbon was a good idea the implementation had a number of flaws that made me question of UX really was the reason and not something else.
My go-to example is how the File menu was hidden behind a non-button that looked just like a fancy application logo.
Why would an actual, caring ux developer do that in a flagship product?
Was hiding the file menu items behind a round logo that looked nothing like a button part of that improved UI?
Is the current file menu that looks like the rest of the menus but behaves totally different part of the improved UI?