You have to consider where tocks fell. The 5, for example, didn't fare so well without an update for 3 years. There is quite the gulf between the processing power in the 5S and the 5. I took my 5 up to iOS 9 beta and it literally died. It struggled on 8 as it was, but I figured that it didn't matter since I was getting the 6S that year anyway.
Good points. I've been on a two-year "s" cycle since the iPhone 3 (skipped the 6s). Which I think is the way to go. I'm going to get the 7, and skip the 8. I like to get phones after the first iteration has worked out the bugs.
I also work on a new iMac, and the thing flys. Though, I can't for the life of me understand why it ships with a 5400 HD. Waiting for the next Mac Pro update.