I also strongly dislike this change and that it was a forced option. I feel if consistency was the goal, it would make more sense to roll out with new accounts only and to give a walk-through with the choice to change it for existing accounts. I think seasoned users prefer customization and have set up their work spaces to be most efficient to them and their preferences. That being said, customization and choice makes more sense than force resetting a default for all users.
I also feel like office.com is the poor copy of the desktop apps (and even less appealing than Google Docs), and I think it's unwise to make the full program resemble something that has been optimized for limited cloud use--very cart before the horse and form before function. I do not feel the ability to add other apps to the sidebar is useful considering the wild limitation for only proprietary apps, none of which my organization uses. Plus we're in some sort of hellish matryoshka of apps in apps instead of well-built integrations. It ain't it in Windows. It ain't it in Office.
While I acknowledge it is more work to make more customization, it would definitely save time and negative customer opinion. I know personally, in the choices I make as a consumer, I am more likely to choose Google over Microsoft for productivity apps, so when Microsoft tries to make Office more like officedotcom, I do not feel like I benefit from a more cohesive experience. I feel more angry that my fully fleshed out program that I paid for is forcing me to find the hidden option to make it look like what I was comfortable and fluent in using, along with making it some weird Rube Goldberg app chimera monstrosity. And that's for all Microsoft. Maybe listening to your customer base and paying myopic CEOs is a better idea than laying off thousands of the people who can build products that people actually like.