"million of users freely decide to pay extra for Windows or even more extra for OS X."
Customers opt for Windows because they don't feel technically proficient enough to handle Linux. If they buy OSX it's because it's the best.
There still is a need for a viable alternative to the monopolistic Windows.
How is it a monopoly if there are alternatives? You've already noted two. You're apparently just whining like a liberal because you don't like Microsoft.
Wow. So you're saying that most people are too stupid to choose anything other than Windows. Nice DNC channeling you have going on there.