Remember, Windows has also been on an upgrade treadmill.
The changes between versions of Windows may not be as big as they once were, but DOpus does need to keep up, if only in the 'visuals'. (Hang on, does DOpus 'do' visuals?)
Changes in Windows are often in areas where DOpus is most active and useful, menus and folders and stuff. It has to keep up there or it dies.
For a real guide to the churn factor, I'd map DOpus versions on to Windows versions.