"maintain" meaning keeping it somewhat workable or actually improving it?

ATM windows still has enough of a moat that they can comfortably do the former.

I believe Microsoft can skate for a long time with just bug fixes and security updates. It makes the drop in Windows' quality all the more baffling.

They literally tried that strategy with Internet Explorer 6 a long time ago where they didn't really update it for years, only doing the bare minimum. The result was a downward spiral in market share that they were unable to stop once they started trying again, ultimately resulting in IE effectively becoming obsolete.