I definitely think the skeuomorphic designs of iOS4-6 were the best, with the Mac using it too. Those were the height of when Apple really had good fun design. Now it's just trying to be ultra-modern, or ultra-current, which is fine, but it's waaayyy boring, like others have said.
But when it comes to iPhone decline in sales, I definitely think with the implementation of 128 GB device sizes, this made TONS of people go from every 2 to every 3 or maybe even 4 years apart. I knew when I got my 128 GB iPhone 6 Plus, I bought a sick arss hand made leather case, and was like 3 years minimum! Now I am looking at iPhone 7s Plus, iPhone 9 Plus, etc every 3 years for sure alternating between non-s and s-brand, like Kevin Hart said, "it's gonna stick."
One other reason to stay with an iPhone for 3 years or more, is I just like having my iPhone taken care of I don't want to have to go thru the purchasing and new setup over and over, and new contracts, etc. It's nice to just have this all done, every 3 years is ok. I don't even do beta releases for iOS or Mac anymore, just no more time, to re-install, run thru iTunes copy over 80GBs etc. I once said the best OS is one that you can have installed on a machine, update, and it keeps working for more than 2 or 3 years, no re-installs needed, we're getting there with iOS devices too.
Laters...