ChrisP: Microsoft are dominant for completely different reasons. Also remember that the situation is quite different: At the moment, the only way you can buy and run RISC OS is by going to CTL. Before, there were many choices, both for the hardware and the OS. Microsoft don't have that amount of control over the PC market - they don't mandate who you buy the hardware from at all. Apple are a much more accurate example. When they started licencing out MacOS to third parties, they suddenly noticed that they were all undercutting them. They couldn't compete. They couldn't make the machines cheaper. So they just withdrew everybody else's licence.
I personally strongly believe that the OS should be maintained by a company with no interest in hardware.