I agree that some markets have a greater tendency for forming natural monopolies. I think the reason for the desktop OS monopoly tendency lies in the fact that

I agree that some markets have a greater tendency for forming natural monopolies. I think the reason for the desktop OS monopoly tendency lies in the fact that developing software and drivers for multiple operating systems is expensive. This, and the desire to have marketable skills, drives developers to one OS. And also, applications you do your work in were typically incompatible between platforms, thus creating a drive for users as well to band together on one OS... Some markets have a huge tendency for natural monopolies, perhaps mostly due to this cost of switching (or sitting in two or more chairs, so to speak). But even in a market where you can easily switch one supplier for another, there often remains a helpful factor for grouping - everything costs less in large quantities ;) And many administrative costs can be reduced. Not to mention that perfect competition would, strictly mathematically speaking, drive profits towards zero...