A ground-up rewrite for... window snapping? :-)

I joke, but it's important to remember just how far behind Windows in basic operating system architecture Mac OS 9 was when Apple launched OS X. Apple absolutely had to replace Mac OS from the ground up. It had no choice.

Is macOS (the artist formerly known as OS X) in the same position today? I would argue no. Most of the "problems" with macOS are either UI complaints like window snapping, or ugly bugs--which are more likely the result of development workflow issues than fundamental shortcomings in the technology of the OS. When Microsoft made the big shift to address their security issues, it was mostly a sea change in how they developed Windows, not a ground-up rewrite of Windows.