«The iPhone 5S gets a little slower, but that's how it goes when the oldest hardware that runs an operating system is only 20 or 25 percent as fast as the most recent hardware»
So it's not like it's due to some Apple mischievous behavior, it's just how things go.
And my response to that, yet again, is: no, it's not "just how things go." It's how things go with iOS. The same thing does not happen with macOS, with Windows, or (I'm told) with Android. My 2013 rMBP is as fast or faster on 10.13 as it was on 10.9, which it shipped with, yet a 2013 iPhone (a 5s) performs far worse on iOS 11 than on 7, which it shipped with (documented in a video posted a few pages back comparing iOS 8 to 11 on a 5s).
This idea that slowdowns are inherent to OS upgrades is nonsense, and it needs to die. The basic UI of iOS is a grid of icons. There's absolutely no reason it should be slowing down to this degree, or at all, when the A series chips are now rivaling Intel's Core family in raw power.