My hypothesis is today's "modern" OS user interfaces are objectively worse from a usability perspective, obfuscating key functionality behind layers of confusing menus.
It reminds me of these "OS popularity since the 70s" time lapse views:
https://youtube.com/watch?v=cTKhqtll5cQ
The dominance of Windows is crazy, even today, Mac desktops and laptops are comparatively niche
These kinds of things almost always give me an uncanny-valley feeling. Here I'm looking at the screenshot and can’t help noticing that the taskbar buttons are too close to the taskbar’s edge, the window titles are too narrow, the folders are too yellow, and so on and so forth. (To its credit, Wine is the one exception that is not susceptible to this, even when configured to use a higher DPI value so the proportions aren’t actually the ones I’m used to.) I’m not so much criticizing the theme’s authors as wondering why this is so universal across the many replicas.
The problem is that the interfaces these bootleg skins draw "inspiration" from were designed on the back of millions of pre-inflationary dollars' R&D from only the best at Golden-Age IBM, Microsoft, Apple, etc.. BeOS, OS/2, Windows 95-2000 do not look the way they do because it looks good, they look the way they do because it works good, countless man hours went into ensuring that. Simply designing an interface that looks similar is not going to bring back the engineering prowess of those Old Masters.
In modern times telemetry can show how well new designs work. The industry never forgot how to measure and do user research for ui changes. We've only gotten better at it.