This is just PR speak to cover for the false dichotomy that you must accept new features (and regressively slower OS upgrades) to receive security updates - or be vulnerable. In reality, Apple could easily back-port security fixes. They are abusing security to force undesired changes to functionality.

Edit: Take a look at how any Stable/LTS Linux disto handles this. If a bunch of hackers can do it on such a diverse software stack, surely the company with the largest cash reserves in the world can figure out how to do it on a software/hardware stack they control completely.

I'm not talking about minor releases here... Avoiding an upgrade to a major release (iOS 10) triggers an unavoidable antipattern: Every 24 hours iOS asks if you want to upgrade now or remind you later... but that's not the worst part... At times it will show what looks like the lock screen but if you enter your passcode the OS will auto-upgrade the next time you are plugged into power and on WiFi. If you aren't paying attention, you think you are simply unlocking your phone instead of authorizing the upgrade. I have no problem patch releases, but I find that major releases over time eventually bogs down older hardware.