macOS' problem ironically enough I think is that it is not a cash cow. Apple stopped charging for it with 10.9, but of course more importantly, the Mac itself does not generate a ton of cash for Apple (which was always the true source of revenue). Not a day goes by that I don't imagine features I wish macOS did for me. The desktop OS is not "mature" or "complete", its just not a big enough piece of the pie with comparison to iPhone for Apple to invest time in it. If you were to gift the Mac/macOS business to some random startup, there is a good chance it would see huge stability and evolution. Unfortunately, the business is coupled to a company that happens to make a different product that makes it way more money. If anything, Apple has been removing "features" from macOS at great pain to its users -- and I don't mean 32 bit or this security stuff, I mean the fact that they don't focus almost at all on the surrounding ecosystem. Apple used to continue investing in software for macOS like Final Cut Pro and Aperture, things that kept them invested in the OS itself. They very clearly do not want to be in that space anymore, so its weird to think that the OS is somehow "done" and just "spinning its wheels".
For a good non-open source example, look at Photoshop: it's been around for almost 30 years and people still think its worth paying for and are generally happy with the updates they get. Perhaps some are unhappy with the pricing model, but I actually see lots of people that are very happy with that too (myself included).
macOS only seems stale because it hasn't changed meaningfully in so many years, not because it can't.
With regard to the bugginess: yes, I think it has everything to do with the fact that they are now trying to ship a new version of iOS, iPadOS, tvOS, watchOS, AND macOS every year on the dot.
I'd love to see evidence otherwise, but I do think that desktop interfaces are pretty "done" at this point - you can have a couple different paradigms and layouts, but usually it's the same thing - either a Windows 7-style taskbar/start menu layout, or a macOS dock/statusbar layout. There's not been much change in the desktop space in a long time. Features here, animations there, but desktops are relatively stagnant. The cool thing is that you can extend them with other programs if you want, but mostly they're just a way to launch programs. Not a lot of messing around is needed if you ask me.
Is this a product of Apple's functional structure? Most companies are divisional, so you'd have a certain number of engineers dedicated to MacOS, who have to come up with new features to ship every year, but Apple's divisional structure makes it easier for those engineers to get pulled over to work on the new iOS feature?
For a good non-open source example, look at Photoshop: it's been around for almost 30 years and people still think its worth paying for and are generally happy with the updates they get. Perhaps some are unhappy with the pricing model, but I actually see lots of people that are very happy with that too (myself included).
macOS only seems stale because it hasn't changed meaningfully in so many years, not because it can't.
With regard to the bugginess: yes, I think it has everything to do with the fact that they are now trying to ship a new version of iOS, iPadOS, tvOS, watchOS, AND macOS every year on the dot.