You have a much higher view of MacOS than I do! I would never think it is capable of satisfying "just about any requirements" -- there're so many things people need or want from their computers I don't even know about.
As far as managing multiple windows, I find that once I have more than 3-4 windows open on a single monitor, I start having problems finding the one I want. Sure, finding it is a matter of seconds, but it interrupts the workflow and my thought process, so I often find the window I was looking for, then have to pause to reorient myself to remember why I was looking for that window in the first place.
Is this a first world problem? Maybe so. But isn't most of the things we use computers for first world problems? And if you are concerned about Apple spending effort on what you think is a redundant and unnecessary system, sorry that ship has sailed -- they've already devoted considerable effort and resources into this.
To me, this isn't redundant or unnecessary, it's a system I wish I'd had when I was writing research papers in college and grad school. I never found a 2D computing system that was better than handwritten notes on note cards, or sticking post-it-notes all over a book. 3D computing might finally make that process digital.
So, now that this AVP device is coming MacOS sucks?
Wow. Okay.