I agree with your sentiments, but not your timeline. The mid-'90s was the high point for GUIs, with Windows 95 nailing it pretty much across the board.

And as you note, "flat" design is NO design. It's total dereliction of the design task. Fortunately we're seeing some steps back toward legitimate GUI, where controls are occasionally demarcated as controls.

A great example of Windows's pathetic regression is "dark mode." Since the early '90s (and I mean '91 or '92), you could set up a system-wide color scheme. Inverse color schemes were an unfortunate vestige of the late '80s, early '90s... the advent of the Mac, "desktop publishing," and the effort to make the screen an analog for a piece of paper. That analogy fails.

The result was millions of people reading black text off the surface of a glaring light bulb all day, every day. The first thing I did was set up a charcoal theme in Windows, pretty much exactly what all the "dark" schemes are today. And all properly written applications inherited it and all was good.

So... just in time for people to realize that this was the way, Microsoft REMOVED the color-scheme editor from windows. Only to have to hastily slap a hard-coded "dark mode" back onto the OS. So damned stupid.