I don't think it is nostalgia, I'd say that Windows 2000 was the peak when it comes to usability, and XP, Vista and 7 made it prettier, taking advantage of improved graphics hardware. And then Windows 8 arrived and broke everything.

It is easy to find the culprit: tablets, and mobile devices in general, and to a lesser extent, web apps.

Mobile devices needed a new kind of UI, something suitable for small touch screens. People tried things, apparently they are still trying, because it is a mess.

But now, we have another problem. Desktop UIs and Mobile UIs are very different. And this, in itself, is a bad thing. Mobile and desktop computers do a lot of the same thing, you shouldn't have to learn how to do it twice. So they tried to unify, and in the process, lost decades of UI refinement.

The next issue is portability. We have web apps, Desktop UIs, Mobile UIs, different OSes, etc... Developers don't want to make a different UI for every system, and users want consistency. The least common denominator is the web browser, so designers use that, but the web is made for documents, it is terrible for apps, but clever devs make do.

So as you see, we now have a lot of hard problems to solve at the same time, and the desktop UI became part of it.