I've written it elsewhere, but it might be worse repeating here: OS X is not like some third party program that works for ages and you decide whether to bother to upgrade to the next version or not. It's more like modern browsers: it's evergreen and you're supposed, if your hardware supports it, to be running the latest version. Regardless of any major new features, a new version gets the security updates going forwa…
I think a lot of people might be much happier if they just stuck to what was installed when they bought it. You don't have to worry about performance degradations, things in the UI moving around, new bugs, etc. People complain about their phones becoming so slow and unusable they feel "forced" to upgrade. If they stuck with the version of software installed when they got it, it would perpetually perform the same. In the old days Windows was an exception to this. As you used it, it would get slower, required routine maintenance and seemed to benefit from the occasional re-install. Phone calls, e-mail, audio playback, twitter, basic browsing, and the fitness app you might use doesn't really need the latest version.
Like you mentioned, you miss out on modern security updates. The other big thing (you also mentioned) is buying a new shiny device and finding out you need a newer OS to support it.
I tend to update everything, but I'm also willing to take the time to read and debug so I know what's happening inside and out. I don't blame people for not wanting to spend that energy or deal with the hassle.