mark gonzales (former apple chief for sw) makes a good point why apple has not really been that innovative lately.
But I think the market share discussion is missing the primary calculation that must go on in the exec staff of Apple (and I know if did through the 80s and 90s.) It takes development dollars to keep an OS on the cutting edge, and marketing dollars to explain the differences and advantages. These costs must be recovered, in the end, by sales (thus from customers.)
Market share comes into play when you divide these costs by units. To put some numbers on it, if Apple has 5% share, and Microsoft 90% share, Microsoft can spend 17 times more on R&D than Apple can, and maintain price parity.