The web/webapps/mobile helped lead to this era of "desktop PCs" (to include laptops) where the browser mattered more than the OS. It allowed Apple to become resurgent in the desktop market because OS compatibility mattered less than ever. It's not weird that it led to Apple regaining _some_ market share because clearly there was demand for the Apple/MacOS/OS X experience that may have been tempered by incompatibility…
As an Apple Stan, I don’t understand what you’re referring to. MacOS is light years ahead of Windows in so many arenas, I’m not really sure what you could be referring to when you say “stagnation”. Every OS ever released has issues at launch, not sure if that’s what you mean by “regression”. But man… windows has been garbage for the better part of 2 decades now.
Lots of things irk me about macOS UX. Finder's lack of tree view sidebar really irks me. Having to disable the silly animations and sounds when I get a new machine irks me. The absolutely terrible window tiling system irks me. When I minimize a window, I can no longer tab into it. The settings dialog's weird behavior with respect to resizing on both axes irks me. Can't use 3 monitors without an expensive DisplayLink dock and the secondary monitors end up with limited refresh rate options. Meanwhile, I can just plug just about any dock into my 5 year old Windows laptop and multiple monitors just work. Still can't find anything as good as IrfanView (as old and dated as it is, it made working with image libraries a breeze).
Finder and the poor external monitor support somehow irks me the most because now I end up typing into the CLI 90% of the time because the navigation experience is so bad and for me this is a work machine and the difficulty in using 3+ monitors is silly.
I get being an Apple Stan (love the hardware), but the software UX is 100% bottom of the barrel stuff. Basic OS stuff like Explorer is just light years ahead of Finder.