That'll be because Windowserver now uses Metal 2 to render UI, so if Apple's claims that it's 10x faster than Metal 1, then that's probably where the UI performance improvements have come from.
I personally own an entry-level Early 2013 Retina MacBook Pro with a i5/HD4000, and the iterative improvements they made with each major release really shine through. A machine that was previously described in every single review as underpowered on that department especially WRT Mission Control can now handle any number of windows I throw at it at a steady 60fps, including those blurs and transparencies which I previously had to disable back in the day, and even when compiling, say, GHC. As an extra bonus, the maximum video RAM on that machine was offset from 1024MB to 1536MB at some point (either Mavericks or Yosemite). Similarly, the Mid 2014 one (i7/HD5000) I use at work can handle the main display as well as external screens (including an ultrawide) without breaking a sweat. So much for planned obsolescence.
To add to this: the Nexus 5 was released a month after the 5S. That phone got its last official update last year, I believe. It was also discontinued a full year before the 5S.
Granted, the 5S started at 4GB for 500 USD and the Nexus 5 started at 16GB for 350 USD.
Still, it's hard to support the "planned obsolescence" argument.
How does your argument about Android vs iPhone affect my Mac vs iPhone argument? Macs have taken a performance hit with Yosemite, but have improved ever since, while old iPhones become more sluggish with every release.
I would generally expect new features designed for newer hardware to run worse on older phones. However, I'll agree that some releases are less about features and more about stability and performance.
Admittedly, I can't really speak to how older phones feel after some of the updates. The oldest iPhone I have is the original iPhone 6 and I haven't tried it on iOS 11 yet (currently using the iPhone SE, which _seems_ to run better on iOS 11).
I don't see how you can really say that sluggish is _worse_ than N/A. Worst case scenario, you just don't update, which is no worse than not getting the update in the first place.
Edit: clarity.
Edit2:
I realized I'm not really addressing your point.
I think there are some pretty big differences that make it hard to compare phone OS releases to computer OS releases.
Mobile devices have a much smaller margin for performance. They don't handle multitasking terribly well. These two things mean that the OS doesn't end up affecting the performance of a phone as much as apps and websites do.
One poorly developed app can destroy the performance of the entire phone (even without the app running in the foreground). None of this is true for a non-mobile device.
I definitely wish we would see more performance-focused iOS releases, but I don't think it has gotten to the "planned" obsolescence point as much as just "regular" obsolescence. Hard to say.
It could be less planned obsolescence and more that iPhone/iPad hardware has gotten many times faster over the past few years while Mac hardware has improved by 50% at best.
