All these are viable, and I use a handful of them- however, there's a lot of software that's windows exclusive, and if you're in a corporate environment chances are you're using a bunch of webapps that are IE exclusive. If you're a gamer, graphic artist, modeler, etc, you need directX.

Now, this is all becoming less of a problem (thanks vulkan!) but I would safely say MS has a monopoly still.

Well, for businesses that have invested and in a lot of ways locked into MS, its tough to move off.

For home consumers, definitely, its much easier.