I kinda wonder what if Microsoft (or any major software company) would give their engineers a few decades of time just to refactor their existing systems, opposite of piling on new features. Will it result a much cleaner code, better architect, and stable product?

The last Windows I daily use was Windows 7 Home (same era with Windows Server 2008). The newer ones don't really improved much featurely from my (user) point of view, and from what I've heard, it's getting more sloppyer every newer version.