That's a rather technical view of the situation. The fact is that a lot of applications are only available on one platform, and basically none is available on all. So quite often your choice of OS is determined by the applications you want to run, whether they're games, a certain office suite or nifty productivity apps. This in turn then ties you to hardware, from a very restricted set (Apple) to a little more relaxed one (Windows) to almost anything with transistors (NetBSD).

In addition almost every platform creates its own culture and environment. Linux and the BSDs embody open source software. Microsoft basically is synonymous with office and enterprise. Apple caters to designers and has a pretty thriving shareware culture, with a lot of small utilities due to its history and the fact that it somewhat dominates the laptop space.

So yes, I'd argue that OS, hardware and third-party software are very hard to separate and you have to consider the whole situation when you think about switching. (In my opinion this got quite a bit better within the last decade, though)

In my specific case, while I feel that Apple is neglecting both its OS and its desktop offers, and I think that its competitors are catching up a bit in the OS space, the whole culture/3rd party thing still would play a major role and could prevent me from switching altogether. Just like someone would like to switch to Linux, but if you're working with high-end CAD software all day, that's a bit hard to do. That's probably the far end of the "switching" spectrum, basically prohibiting it, while reliance on neat productivity tools like Alfred, TextExpander and OmniFocus just makes it harder. (And for a lot of people it's not even just the specific apps, but common general qualities, whether that's clean, simple, good looking apps on Macs or being able to configure everything in text on Unices)

I might have stated it a bit awkwardly, and I hope this clears things up.