Running an older iOS is okay as long as your apps will still run under it. I noticed that some of my newer apps require iOS 8.x or greater to run, some of the older ones require 7.x or greater, etc. If all of your current apps still run, and you are happy with whatever iOS version you are running, then there is really little incentive to upgrade. iOS upgrades tend to slow down older iDevices and they typically take up more storage space.