I'm not complaining about the visual appeal of the UIs, I think they look great. I think MacOS is a good example of iterating on your UI experience and improvin

I'm not complaining about the visual appeal of the UIs, I think they look great. I think MacOS is a good example of iterating on your UI experience and improving it, but not changing it to the point where half your OS ends up looking out of place due to tiny inconsistencies like the colour of titlebars or because the entire style of your OS keeps changing upon every new version. Imagine if the next version of MacOS had square icons for close/minimise/maximise and to update your app to use that new style, you had to switch UI frameworks. Then you'd end up with 2 different styles of apps on your system because not every app out there made the switch, and one would now look very out of place. That's the sort of thing Windows seems to do every major release.