I thought Jobs quasi answered this at D8 with the cars and trucks analogy. Mac OS X (and desktop & laptops) will exist for the foreseeable future but they will be relegated to those who actually use more of their functions. A lot of people who go buy a dual core PC with 4GB of ram and a 500GB hard drive are using it for music, movies, the web, Facebook, email and that's about it. These are the people they expect to transition to more iOS based devices, as it fits their needs better.
My mom would be 1000x better off with an iPad than an iMac. And there's almost nothing she would try to do that the iPad isn't capable of. Now imagine as the capabilities of the iPad expand that market will keep getting bigger and bigger.
We, the techies, will have our desktops and laptops but the mainstream market will push towards simpler, touch-screen based devices. Content creation, video editing, photo editing, programming (gotta have a Mac to make those iApps don't you?) and similar tasks will keep the Mac alive for a while.
The computer market is basically splitting between heavy end (creation) and bottom end (consumption) and for 80% their future PC will be a consumption device.
Just my 2¢ and my interpretation of what Jobs said at D8.