True, but each of those updates generally makes the phone less usable from a performance perspective. My iPhone 4 is barely usable with the latest iOS update.
iOS7 in particular was a troublesome upgrade for me. It had issues with my 4's (and my wife's 4S) proximity detector. I would end up face dialing in the middle of a call. And that's just one of many annoyances that came with upgrading to iOS7. I would have been happier if Apple cut off updates after two years.
Compare that experience to my Moto G (that replaced my 4), and each of the updates I've gotten (albeit in a short period of time) have had real performance improvements, unlike the mostly cosmetic animation "speed improvements" that came with the last iOS 7 update for iPhone 4.
