Mac and Linux have such a small market share that they're really no competition. Microsoft used to threaten to pull support for PC makers like Dell and HP if they offered alternate OS's like Linux. Doing things like that, they have a stranglehold on the OS market.

If either OS were a viable option, they would cause prices to drop, better features to be developed, more stable builds, etc.

Less than 10% of computer users use Apple's OS. Apple would have to have closer to a 30% market share for them to be any real threat to MS.

Don't get me wrong, I like Apple and I think they'll be my next purchase, but they just aren't any real competition for MS.