Problem with that expectation (at least for older generations) is ARM has still been getting 1.5-2x performance bumps between generations. After 2 years worth of iDevice upgrades, you'd already have somewhere between 300-400% processing power of your old iDevice. Apple would either have to dumb down iOS updates significantly so devices that are essentially at 10-50% processing power of the latest model can cope well or they could just flat out not release a major firmware update for devices that are merely 2-3 years old. Granted, the option to downgrade would sure be appreciated. Mind, with A8 matching up to Atom and A9 matching up to Core M, I reckon we're close to or already at the level where further iOS updates wouldn't tax the CPU/GPU for UI and most tasks bar gaming, photo/video editing, etc.Apple can do whatever they want with downgrades (I'd like them to allow that, but it won't happen). The mistake, though, is shattering the performance with every single iOS update of non-latest devices. Even on one major version. iOS 9 is bad on iPod Touch 5G, iOS 7 is better but heaps worse than iOS 6. Same applies for iPad 4, for iPhones, for everything.
So you either allow downgrading or test the heck out of older devices and guarantee trouble-free performance. But the first update cannot shatter performance as it is currently doing. I shouldn't fear updating because I don't know if my devices will be a lagfest if I actually update. If Apple allows updating, then it should work flawlessly. Maybe my expectations are too high, but that's how I think it should be...
As for shattering performance, iOS 7 is a bit of a special case since Apple pretty much overhauled UI. Mind, with animations disabled, iOS 7 actually wasn't all that bad. iOS 8 was even "heavier" thanks to new features. I find iOS 9 to be a refinement at least for devices with at least 1GB RAM. It certainly ran better on my iPad 3, 4 and Air than iOS 8 did. I actually found iPad 4 performance on iOS 7 to iOS 9 to be acceptable even if it's not quite silky smooth like iOS 6.
Also, Apple already skimps on RAM but that's even more pronounced on the iPod Touch. The Touch often gets just 1/2 the RAM that a similar iPhone would so it's already RAM-starved at launch, nevermind firmware updates. For the most part, updating to the next major firmware update (on iPhone and iPad) has been okay. It's the second major firmware update where things start to go iffy.
That said, folks on MacRumors tend to be power users with higher performance requirements/expectations. Real world, I reckon most people are more like my mom who is perfectly content even with an "ancient" iPad 3.
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