OSX killed Linux on the desktop indirectly only because the most popular (to the layman) distro is Ubuntu and Ubuntu thought it would be cute to revamp their desktop into a wannabe OSX desktop, pissing many like me off in the process. A Relix is not a Rolex. A Coby is not a Sony. Ubuntu was doing just fine in making a name for itself, now it's a watered-down sissy distro which threw itself at Steve Jobs' feet rather than continue manning the fuck up.
I think Ubuntu's changes had something to do with a drop. I know this from my many IT friends who went back to Windows because they "didn't want a phone interface on their PC". That said, Windows 8 in itself may drive them back. I advocate the position that Gnome 3 (and the way it was handled by the developers) directly and indirectly (causing Ubuntu to go with Unity) caused a large fracture in the Linux community which chased many away.
What really is killing Linux (or why it's never really taken off) is the fragmentation issues. Sure, choice is a good thing but in the present implementation of these available choices full QC (the boring stuff) is rarely done. This leaves a system lacking "spit-n-polish". A good example of this fragmentation is KDE vs. Gnome(2) libraries. Although different under the hood, are they really all that different to the average user?
Where Apple comes in is that "spit-n-polish" that is missing. If you create a quality vacuum, it will get filled. It's as simple as that.