Microsoft feels that it has no choice but to follow fashion trends and I am not sure that they're wrong to do so. Eventually minimalist UIs will go out of fashion and utilitarian evidence-based UIs may come back. But for now, the fashion is what it is and if companies want new users, they must follow the trend.
Regarding the hodge-podge of different UI designs, I think that that is just a function of UI/UX designers not being honest with themselves and others about whether they're doing engineering, psychology or fashion. Fashion must change for the sake of change but engineering and psychology do not. This creates inconsistencies.