Of course, there are so many that are frustrated but don't have time to complain to Microsoft or don't know how. Example, my dad was so frustrated with Windows 8 on his new laptop that he asked me to "make it work like it used to". He probably won't be contacting Microsoft himself (though I go an ear full ).
That huge user base also is split over so many versions of Windows so the number would be much smaller for newer versions of Windows. Windows XP is still hovering at around 40% of the Windows market. Those are users that either have decided they don't want to upgrade and face problems they know about (or are content with what they have).
Also, the data that really changes Microsoft's mind is the data they pull back via the Customer Experience Program. Instead of asking 10,000 already dissatisfied to the point of posting users whether they like Windows8 (which they won't, since if you like it, why would you blog about it - it's an OS and therefore boring), Microsoft can just measure how all of the users actually use it.
For example, your father's dissatisfaction is not recorded in UserVoice (because he falls into the category of people for whom blogging about stuff or writing it online simply isn't a course of action he would consider). But his dissatisfaction and confusion is recorded in CEP, and that means that his confusion holds vastly more weight than every viewpoint from someone who doesn't use Windows8, and exactly as much weight as anyone who either likes or dislikes Windows and is technical enough to say so loudly online.