Again, I agree with you.
However, Apple supported the iPhone 5 with iOS 7/8 for the same reasons they supported iOS 6/7 for the iPhone 4 and iOS 5/6 for the iPhone 3GS.
I had a 3GS running iOS 6.1 at one point. Of course, no Facetime as well as some other options, but it ran pretty smooth on it.
Again, Apple doesn't want you jailbreaking. So "supporting" the iPhone 5 (and the 4s) on iOS 7/8 is one way to prevent that.
The other thing is that unlike us users who prefer iOS 6 the "keeping up with the Joneses" crowd have to have the latest and greatest on their current phone.
I wonder if Apple will bring that textured look back to the OS any time soon. That's what I love.
Anybody know why iOS 8 isn't as fluid, even in good hardware?