I have an impression that they force at least part of those "UI upgrades", to the point of removing their switch in about:config, specifically to annoy users an

I have an impression that they force at least part of those "UI upgrades", to the point of removing their switch in about:config, specifically to annoy users and to create waves pushing their product into some news while showing off some work being done. There is no such thing as negative marketing or so people say. Personally, I found almost none of those as useful as to justify the repeatedly broken web-surfing experience and ever-growing userChrome.css.