There's a big difference between hardware obsolescence and software obsolescence. When hardware "moves on" like Chupa Chupa's DVI HDTV example, there is not much that can be done. In a software issue like this, I think we all know that if the issue at hand was possible, our old iDevices would "feel" like they are as fast as they were when they were new. I think we all must believe to some level that part of iOS upgrades is building in code to make older iDevices feel slower. Else, how else do we explain it?
I can explain this as a software developer. Developers are constantly dumping more code into the OS to add features. This gets even worse when leveraging open source software, because then it's easy to spend one day and dump in 10 years of software that was actually developed for PCs, not phones.
A lot of these new features - such as frameworks or user interfaces - are used all the time. More code = slower response. It takes a *LOT* of work to optimize code and make it leaner. If the target for a new OS is a new piece of hardware, and it runs "fast enough", programmers are unlikely going to be given time to try to tune it. And each OS runs slower. My iPhone 5S was fine even through iOS 9, but on iOS 10 it's like I hit "launch app", then do something else, then come back.
It's not to see that new updates are *never* efficient - a couple of times on the MacOS Apple made a concerted effort to speed it up. And before making the iPhone 5 and earlier obsolete, Apple did some heroic work to port modern iOS features back to 32-bit platforms, and in so doing, made them as efficient as possible.
But as a programmer, even if your goal is to make things "as fast as you can", if you're not developing on a slow system, it's pretty hard to analyze.
I often dream of developing a new platform outside of any deadlines, so it can finally be "done right" - you can certainly make any OS features we see today run at full speed on any old device - you just have to rewrite everything.
On the PC, "downgrading" is an old trick to speed up your hardware - like hitting the turbo button.
BTW, for the most direct example, many of us remember the NeXT computer - and NeXTStep is what turned into OS-X, which turned into iOS. The look and feel of a NeXTStep was almost identical to both Windows 2000 and modern OS-X. And it ran fine on a 5 MIPS processor and 8MB of memory. Now that Moore's Law is almost over for hardware, companies are saying the next 10 years of Moore's Law is about the software - mining back that 1,000x slowdown we introduced by being lazy. Once Apple's A-chips hit a wall, it'll be all about making each OS upgrade faster.