Regarding planned obsolescence, here's my opinion:
Apple used to optimize iOS to the point that it runs so fast and smooth even on weak hardware (I.e: ios 6 on the iPhone 5 is the smoothest os on a smartphone ever, smoother than i6s on iOS 9 and i7 on iOS 10). That was the case until iOS 6.
When developing iOS 7, Apple introduced many new features and design changes and didn't have "time" to optimize it properly, since time and engineering effort is finite, that's why it slowed down older iPhone especially the i4.
But Apple got a little lazy over time.
It is not really "laziness" but they thought: why spend so much money on optimizing the software for such capable devices. So Apple is investing less and less in optimization. But thankfully hardware is evolving faster than the lack of software optimization, if you understand what I want to say, so ios 10 is running much faster on a 5-year-old iPhone 5 than iOS 7 was on a 4-year-old iPhone 4.
This is explained better in the same article under "ui performance" here:
http://www.anandtech.com/show/10685/the-iphone-7-and-iphone-7-plus-review/9
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And what happened with that?
I really don't know
I just remembered that I heard about something similar so I googled it.