I don't think they are intentionally trying to cripple the older devices. Apple is not letting the old devices hold back the new devices. The old devices are lucky to be allowed updates and in such a timely manner. Try buying a Samsung Galaxy phone and waiting for ages to get the new OS version. You might get one when the second new version is released and that is a big if.The cripple as much as legally possible to make you upgrade. You can't have well made 'to last' hardware and have it run well forever... Nobody would be upgrading.
I can see myself keeping my work iPhone 5S and personal iPhone 6+ for a number of years still because they work for me. I still have a 3GS that I leave in the car to play music. I rarely touch it but it is still alive and kicking.