Blue95: a desktop for your childhood home's computer room

This looks nice and easy to use.

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

Microsoft Windows programs hid functionality under layers of menus and the registry. MacOS, at least, surfaces much less functionality, because it offers sensible defaults. I never had to do anything akin to fiddling with the Windows Registry.

I did like some Windows things, though, like the ribbon, and reconfigurable UIs. Today's UIs are more immutable, for the worse.

I'd agree macOS surfaces much less functionality but I feel like it's more "because they don't want you to feel like there is a choice to make in the first place" rather than "because the defaults are ideal for everyone". Over time it feels like "layers of menus" have definitely made their way into Apple's software anyways.

The replacement to the registry seems to half be "magic CLI incantations for settings which can't be found in the GUI for some reason" and half "here's a $4.99 app to 3 finger tap to close tabs".

And the defaults system is just registry by another name

Not really, defaults are stored in per-application plist files rather than in a singular database.

And what difference to end user it makes where exactly the key/value data is stored? No real difference whether the data is HKEY_CURRENT_USER\Software\MyAppName or com.my.app when you're trying to coerce some internals whose configuration is not exposed because you're not worthy of it