Erm. They don't want you to know they're intentionally slowing down your old device. That would be admitting to something terrible, yet obvious.
Evidence: iPhone 7 Plus running latest iOS 11 beta. Yes it's a beta, but we're dangerously close to the end of said beta and there has been a clear slow down of the device. Just in time for...oh, look a new iPhone...mine is feeling sluggish, guess I'll buy the new one! Kerching$$$$$$ (says Apple).
Now, if anyone could prove that Apple are intentionally doing this - that might just be as big a scandal as the VW emissions test falsifying. Could be highly damaging. It benefits Apple too greatly to be mere coincidence. Didn't someone find recently that 'iPhone slow' searches on Google raise 10 fold when a new iOS drops/new iPhone is released?