Interesting that people mentioned Windows. Prior to 7, Windows was well known to slow down existing machines on every major version update. Microsoft was basically relying on moore's law for their OS to perform as intended. The pinnacle was Vista. Although it was actually not a bad OS per se, it's a far jump for most machines running XP (mainly because Microsoft was keeping XP for too long).
Then MS realized it had to do better. Came 7, then 8 (which is faster than 7 despite people hating on it), and 10. Like it or not, give Microsoft credit where it's due.
It also shows that we are not seeing a huge jump in performance every year on intel CPUs.
iOS, on the other hand, is akin to the early days of PCs, where every new SoC delivers a significant performance jump. Plus Apple's obsession with GPU (thus older SoCs with weaker GPUs suffer).
The 6 Plus adds more complexity, considering it is rendering everything in higher resolution AND scaling them back to 1080p, meaning its GPU is doing more work than the 6. Add this to Apple pushing GPU, we can see why the 6 Plus suffers.
I don't believe in planned obsolesce. At the same time, we have to be a smart consumer, especially since we are frequenting forums like this. I take myself as an example. After the iPhone 4, I told myself my next iPhone must have 1GB of RAM, so I skipped the 4S and got the iPhone 5. After that, I told myself my next iPhone must have 2GB of RAM. So I skipped both the 5S and 6, and waited out till the 6S came out.
Buy smart.