Now that Microsoft has stopped selling its last operating system, folks wanting to avoid its new operating system have made versions of its predecessor major players in Amazon's top-25 list for software.
The linked article points out that Microsoft benefits from being ranked even higher in sales for its office suite. I would add that these numbers would be higher if folks better understood that their state university ID cards could get them state government pricing or media for installing personal use under university site licenses (not long ago, that was under $20 at the University of Texas). However, easy things get complicated when Microsoft is involved. For example, if you do a year abroad, do you need a note from a Redmond parole officer?
This kind of complexity, and its attendant compliance overhead, makes competitive offerings all the more attractive.