And it'll never be the same again. When you add new features, you can't expect everything to run at 60FPS. It's just not possible. Even if iOS 10 LOOKED the same as iOS 6 but had all the FEATURES iOS 10 had, it would NOT be smooth. Simple.
Erm... I don't quite know where you got your UI/UX knowledge, but this definitely isn't the case. It certainly isn't 'simple', as you put it.
To suggest that added features results in a degraded UX is absurd, and implies that there is an issue with the way Apple is managing their hardware/software synergy. UX is about
optimisation -- added features should have NO EFFECT on smoothness in any way - older devices notwithstanding. If adding new features results in the NEWEST device, let alone devices such as the 6s and 6, being more sluggish than the iPhone 5 was at launch, then these new features are not ready for prime time and need to be baked a little more.
'It's just not possible'
Why not? Because Apple told you so? That's an incredibly pessimistic way of looking at things and, frankly, apologetic for the unacceptable drop in quality control we've been seeing at Apple for the past three to five years.
This issue is far from 'simple'.