What does Apple even get out of it? I don't think anyone is clamouring for new MacOS releases, so I can't imagine it drives hardware sales. MacOS is no longer a paid product either.

The regular update cadence and the lack of stability for developers sets an expectation that developers must be active (forever) or their product will stop working.

That way, they never have to deal with fallout from breaking a 20 year old program like Microsoft got for breaking whatever version of GTA. There's no way a 20 year old program still runs on current macos, so nothing to worry about.

I worked on some Mac security software. WWDC and that first macOS beta in June dictated whether we were going to spend the summer on features, or on compatibility with the new platform. There were a few crazy years, like file protection in Mojave, system extensions in Catalina, and Apple Silicon and other changes in Big Sur. Not to mention yearly device management changes and UI changes like dark mode, completely reworking the menu bar icon, etc.

The Windows team never faced anything like this. Not only did Windows change slowly and with backward compatibility, but the users didn't upgrade right away, even if they were buying brand new laptops. But in the Mac world, a laptop bought in September/October is going to have the new macOS on it no matter what.

I'm not sure. Perhaps Tim Cook just likes schedules, which fit naturally in his MBA brain. Or it could be that Apple previously set the expectation of annual releases and now can't stop, because getting off the update train would be a kind of indicator or admission to stockholders and the media that Apple is falling behind somehow. On the other hand, annual OS updates with updated system requirements would provide a convenient way to implement planned obsolescence of hardware.

In any case, Tim Cook clearly doesn't have the same standards as Steve Jobs did. Cook will never complain, "This is shit!" As Jobs reportedly lamented, Cook is a not a product person. So QA problems are not necessarily a problem for him. I suspect that Cook actually believes everything is mostly going well, and perhaps some "metrics" tell him so, though the metrics likely ignore the fact that developers have been disillusioned and alienated by Apple's hostile bug reporting system. If I thought Apple cared and would do something, I'd file literally thousands more bug reports.