They do not optimise the newer iOS for older devices. As a result this creates some slowdown yearly. Whether this is noticeable by everyone or not is a different matter. It’s possible majority of people out in the real world don’t notice it. But they don’t need to.
One person buying a new phone because of it is one more phone sold. Any person who does is likely to buy a new phone, usually an iPhone as people are reluctant to switch ecosystems rapidly and use something completely new.
My iPhone 6 is definitely not as a fast as it was on iOS 8(Not even close actually) and I won’t bother replying if you continue to state it is.