Its even worse than that. Apple blocks you from going back to the older version of the OS. I had to literally throw away my 4s, (it was working fine otherwise) because it had become unusable for me after updating the OS.

The primary cause of that sort of thing appears to be the significant leaps forward[0] they've made with their ARM hardware every year.

At some point it will plateau the way PC hardware has, and new major OS versions won't be as much an issue.

[0] https://arstechnica.com/apple/2017/06/review-10-5-inch-ipad-...

On the other hand, my iPhone 5 from 2012 is still plenty fast on iOS 10.