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

What do you mean exactly? Like the Menu-Issues in Windows 10? Because from a UX perspective, basically nothing has change. UI of course, but UX is the same like in the 90's following "The Design of Everyday Things" by Donald A.

I think its more about the change management, expectations. For example in Win XP you had the option to use the NT theme. As a user: "I can decide when to move on to the new design."

Usually around 50% of your users are conservative about change. You have to keep this in mind when u change design. On the other hand, if you sell a product with subscription, you have to introduce new feature, else user will move to another product. But, when you introduce new feature, UI gets more complicated and user will blame you for that.

I'm sorry, but absolutely no. Fuck no. Nothing from Microsoft has even been in the same building as a copy of "The Design of Everyday Things", or as a copy of any old-school UX book from before UX meant "Electron". UX is just as much about the "how" as it is about the "what", and Microsoft has been failing everyone lately on this count.