Agreed—within a few years, they might even take over more than 50% of the desktop market, which would be a phenomenal blow to Microsoft. But they're not going to, because Apple is run by idiots. They guard their dwindling hardware market so zealously that they're perfectly willing to pass up vastly more profitable software sales—just ask anyone who worked at Power Computing. Even if Apple decides to switch to x86 CPUs for Macs, there will still never be a version of OS X that competes with Windows, because Apple will never, ever release a Mac OS that doesn't require you to buy hardware from Cupertino.
Sigh. I know. Personally, I think its silly. What's the diff if Apple sells computers or just the OS? The Cupertino folks probably didn't realize this, but if they released OS X on x86 - the PC people might actually be inspired to give Apple computers a try -- I reckon OS X on x86 would actually increase their hardware sales.