The whole point of using an iPhone is that you don't have to update it?

No, the App Store allowing you to download an app with an extension target that doesn’t match the current system. At the very least there should be a warning or a button to update iOS if that’s possible on the current device.

Choice and platform fragmentation in one neat bundle. I am almost surprised that Apple doesn't just force updates on everyone when it is possible just to maintain that additional bit of uniformity and conformity for its "think different" crowd.