A friend of mine had a theory that gadget performance was designed to degrade overtime to motivate people to upgrade. I doubt that's true but as each update uses more hardware power eventually you do need to upgrade
The reason each update uses more hardware power is that there's more hardware power available to be used. About twice as much every other year, as a matter of fact (Moore's Law).
Oh, and that new WiFi vulnerability that was in the news last week (KRACK)? That will never get patched in iOS9. Hope you don't keep anything sensitive on your old hardware.