Imagine a world in which Apple enjoys 90% market share, while clinging to its hardware-centric PC business model.
It can't happen, of course. If Microsoft constituted a monopoly for supplying the operating system and much of the application software for the vast majority of PCs sold, while only dabbling in PC hardware itself, how much more so would Apple constitute a monopoly if it controlled both hardware and software for a similarly large percentage of the market.
(Actually, Microsoft's still huge market share continues to constitute a technical monopoly, despite viable alternatives, since it can still put the screws to others by refusing to supply or maintain its software for their systems, and can still undermine standards efforts by developing and promoting its own incompatible, proprietary versions — but that's a topic for another time.)
The plain fact is, Apple's going to have to alter its PC business model long before Mac OS X reaches 90% market share, simply because there are too many people who stand to lose too much, who'll cry foul too loudly if Apple should ever even approach that degree of market dominance.
Apple is already trying out alternatives, of course, for example with the iTunes Store and the marketing of iPhone applications, and I expect to see more such experimentation. Moreover, there's a long way to go before PC market dominance is something Apple should have to be even remotely concerned about; they could double their present market share, and double it again, and double it yet again, and still have no cause for concern.
On the other hand, dominance of certain other markets is much closer to being a reality, and it may not be all that long before charges of monopolistic practices start to stick. That kind of PR Apple doesn't need.
I don't have any magic answers. It's a tough nut to crack. There are good reasons why Apple behaves as it does, and there's going to be risk involved no matter which way they choose to go. I just hope they don't wait for some court to force their hand.