After reading through the [current] four pages of posts on this, looks to be around a 60/40 or maybe even 70/30 split of those that keep their iPhones for longer than a yearly upgrade. Yes, lately, they are getting better and better, and last a lot longer.
I was intrigued at the very first iPhone and was with T-Mobile way back when. Still, I bought the 16gb iPhone and jailbreaked and unlocked it. Was fun back then; when iPhone 3G came out, the design didn’t catch me, but I upgraded to the 3GS when speed was the new thing and switched to AT&T.
I then got the 4, 4S, 5, and 5S by selling each prior year’s model on eBay and put the money toward the new iPhone. Generally simple. The digitizer on my 5S died unexpectedly and I ended up going with the annual upgrade program...I’ve since had the 6, 6S, 7, and now an 8 Plus (my first BIG iPhone). The only reason I didn’t wait the extra few months was the X didn’t come in gold as I’d also just gotten the Series 3 Apple Watch.
My gifts to myself each year are iDevices...to each his or her own, but I don’t even see the total price of the phone anymore. It’s a small monthly payment and I enjoy the latest and greatest that Apple creates.