THIS MAKES SO MUCH SENSE. I always thought I was going crazy, thinking my increasingly "old" phone seemed slow. This is really irritating and disingenuous on Apple's behalf - as said above, they should at least add a switch.
My smartphone in 2008 was an iPhone. It slowed down unacceptably and I never bought another piece of Apple hardware again. My 3 phones since then all remained usable past 2 years until I was ready to replace them on my own schedule.
The iPhone 5s should have hardware that is much faster than what's in a Moto G2. Yet, my friend's Moto G2 feels smoother in use, transitioning between apps and the like, than the 5s. I bought the Honor 8 to replace it, a somewhat cheap, midrange android device. Everything on it feels so fast I probably won't replace it until its battery gives out completely. On the paper, the hardware of the Honor 8 is not as fast as modern iPhones. In practice when I open apps it's pretty much instantaneous. The device has enough RAM to keep everything running.
People keep complaining android manufacturers aren't updating the OS fast enough. I'd say good on them to make sure they only give us an OS build that's actually usable on said hardware. Apple doesn't even let you install previous iOS versions. Unlike iOS, most of Android's platform APIs are updated through the Play Store. Things like your web browser are also updated through the play store. So unlike iPhones where refusing new iOS updates means being stuck with browser engines that can't keep up with the web, your android handset stuck on older android is not actually becoming obsolete.
No matter how much nicer Apple's hardware looks and feels in the hands, I ain't ever giving them more of my money again. Premium prices should command more durability in time than this. Chosing to stay on an old iOS means you can't install new apps built on newer SDKs or get newer browsers and so on, so you really have to suffer the slow down treadmill with Apple. You don't have to on Android.