I wonder how much of this is a lack of decent alternatives which are well-marketed, versus simple cultlike behavior.
Think about it: if you want a PC-like computer, what are your choices? There's Apple which is well-marketed and attractive and shiny (but has downsides which aren't as obvious unless you're really familiar with the industry), there's Microsoft-driven PCs where you have to use shitty Windows and deal with all its annoyances, and there's Linux where there's pretty much no one actually pre-installing it and marketing it to consumers (plus there's a ton of different graphical environments, and the two most publicized ones are radical departures from Windows/Mac and have a lot of usability issues, and the one which is a great fit for Windows converts is simply ignored by the Linux community for some reason).
Maybe if there were another company making PCs with an attractive, stable OS which was very usable and could market it decently, we'd be seeing more people abandoning Apple.
This analysis of course is for PCs, not mobile devices. But over there the situation isn't much better; there's Android which has huge marketshare but still you have to deal with a complete lack of support after you buy the device, and then there's Windows Phone which is tainted by everyone's memory of Windows and MS problems. But still, Apple has lost a lot of marketshare in the phone market (though some of that is simply due to the market expanding, and Apple not aiming at the lower end).