There would be nothing preventing a rewrite of core subsystems. Vista, despite it being a disaster for Microsoft, was only problematic because of changes made to the UX.<p>Actually Vista is interesting. For all its problems, it&#x27;s actually an impressive upgrade - whilst there was development chaos due to a lack of focus, a new management team was able to significantly focus in on what needed to be done and hundred of improvements were made under the hood. Microsoft rewrote their TCP&#x2F;IP stack from scratch and included important system administration tools like an improved logging system, performance and reliability tools, a vastly improved desktop window manager, a new graphics driver model called WDDM and a raft of new kernel security measures.<p>These rewrites weren&#x27;t rewrites of the entirety of Windows, but instead Windows was increasingly modularised and entire subsystems were rewritten. Whilst Vista was a disaster and had some real problems I don&#x27;t think it was because of rewriting <i>subsystems</i>, I think it was due to UX changes.<p>Interestingly, Windows 7 was an amazing success and in fact built on Windows Vista, but again focused on modularisation and leveraging the work done in Vista.<p>My hypothesis that major UX changes caused Microsoft to stumble in Windows is born out by Windows 8 - which was even more stable and performed better than Windows 7, but was rejected in the market because of insane UX decisions.<p>So a major rework can be done, but if you are going to do this then I suggest that major changes to UX should be done <i>after</i> the rework.<p>I don&#x27;t think you can avoid major rework though. Without some of the things I highlight above, there is no way Microsoft would be able to improve their code or implement features that actual make sense!