I think one of the reasons Microsoft is still dominant in the desktop market is to be found in businesses. They provide a much better integrated software suite for office work and system administration, all packaged together in a way that just works, and with minimum effort. The specific tools are usually not better than the equivalent open or linux compatible versions, but the convenience of only using proprietary stuff is that you don't have to care about messing with software not made to interact.
I personally think this is no longer true. There are good linux-based bundles of tools that work the same way you are describing. Linux still has the problem that hardware vendors don't care about it, which means new hardware will generally not work well, until some third party writes a driver and basically does the hardware vendors job (which might never happen). But if you get lucky, and everything works, Linux based software can provide an even better user experience for office work and administrators.
However, as stated previously, this is not even the main problem. The main problem is that people want Outlook, not Evolution or Thunderbird. They want Photoshop, not Gimp. Powerpoint, not OpenOffice Draw. Most people are not willing to relearn these tools. Also, most people find choice intimidating. Many have told me that they don't want to decide between Banshee, VLC, Kaffeine, Totem, Mplayer and what have you, they simply want a media player. Strangely enough, they have no problems doing the same thing with commercial software, maybe because there is a price tag attached, that helps in the decision.
