Great easy to force obsolesce then. Old phone forced to run new bloated OS.
That said, iDevices generally have a much longer useful life than Android devices (considering when the manufacturer stops releasing major updates). If you got an iPad 2, iPhone 4s, or Apple TV at launch (all A5 devices with 512MB RAM), you had an excellent phone that had the latest updates and apps for a
very long time. The early 64-bit devices felt a performance crunch due to low RAM capacity and slow storage. iOS 10.3 updated the storage to APFS and should have significant performance improvements, so it shouldn't feel as bad when it has to use the storage to swap what's in RAM.
I expect iDevices with 2GB+ RAM to remain useful for a very long time. Apple has incentive to keep those hand-me-down devices going so they can keep making purchases from iTunes and the App Store.