I've bought a Macbook Pro a pair of years ago, and it came with OSX 10.9.5 (Mavericks). Since I am very happy with this version of OSX, I've ignored all the updates since then, and I'm still running OSX 10.9.5.
I'm afraid that upgrading OSX would leave me unsatisfied, so basically I'm sticking with a version I like (and by the way, I also prefer this version from an esthetic point of view!).
However, I'm wondering if there could be some problems when never upgrading OSX. What are the problems that could emerge in the long run if the OS is not upgraded?