I am still running my Lumia 630 running OS 8.1. It still does everything I need it for (make calls, run HERE maps and a limited set of business related applications, access the web if I am in dire need of info and don't have access to my PC). It still has extremely good battery life which is about the most important factor for me on a phone. I turn wifi and mobile data off when not in use and only need to charge it once a week. The camera is OK, but for serious photography I have a DSLR.
I can't see the appeal of playing games/watching video/serious web browsing on a tiny screen, so can leave that until I get access to a device with a proper screen.
Obviously other peoples use cases are different. This suits mine. I can't see any reason to change it.