This, in my opinion, is most likely due to Apple's constant push for the best possible security and privacy on their devices. They are worried about older software being exploited (i.e. Jailbreaking). This is also the reason why most of the time, iOS doesn't get 'widespread' malware attacks targeting the entire ecosystem. It shows they care about their users, while at the same time unfortunately, causing inconveniences for some (typically those who refuse to update their devices, either for keepsake or just generally those who have a negative perception of updates).
Oh, and it makes the little pie charts they always show at WWDC look nicer.
I agree with you on that, it's an extremely good thing to have everyone's devices as safe as possible. (personally I'd leave jailbreaking to do its thing, as long as apple keeps buying their exploits, so they don't sell it elsewhere for hacking privileges).
I think what it comes down to is how you perceive your device. I forget that most people use their phone as a machine to get things done with. Similar to people who buy a very generic car like an X or Y. They bought it to get from point A or B, and they won't notice if the steering gets a little misaligned, or care to rotate the tires on a regular basis. For a power user, they will notice, and get annoyed by changes that downgrade their experience.
I'll leave it at that, I do agree that the issues inherited by their approach to updating affects a relatively small segment. However I do believe that people simply learn to endure their newly sluggish devices without complaining too much. They should do a test of Cortisone levels after people update their iOS devices
