Two words: planned obsolescence
This has been going on since the iPhone 3G. People say "new APIs" or new features or whatever garbage excuse they allow Apple to have.
The fact is my Windows 10 PC runs as well, if not better, than when I bought it with Windows 7. Windows 10 has tons of new features, a new look, and maintains the same responsiveness on older hardware. The difference is that Microsoft cares more about its software sales, a lesson learned with Vista's bloat. Microsoft keeps things lean and fast to sell you its latest and greatest.
Apple no longer sells their OS. They sell hardware. They want you to keep buying their hardware. Anyone who believes otherwise is deft, and trust me, there are plenty of you out there.
Next year the same thread will be created for the 6, then 6s, then 7....etc. Even the unreleased iPhone X will suffer this fate. But I know everyone here will lose their minds when Apple shows off the iPhone X benchmarks....only to turn it into a turd in 4 years because of "new API's". Comedy, I tell you, comedy.