Yeah, that must be it. Didn't the Air 2 and iPhone 6 ship with iOS 8?
The planned obsolescence theory doesn't hold up. First of all, clearly not everyone experiences this "significant slowdown". Secondly, when most people upgrade their phones has typically been tied to the conditions of their carrier contract more than anything. If you're in a two year contract, chances are you aren't buying a phone for two years. The only thing purposely diminishing the performance of a person's phone will therefore do is make it less likely that they will want another iPhone when it's time to upgrade. Apple and carriers are even trying to get consumers into yearly upgrade plans. Planned obsolescence makes no sense when most your customers have already agreed to upgrade on a predetermined schedule anyway. Which brings me to the third point; Apple's biggest advantage is the perception that iPhones are perhaps the smoothest phones, offering the best user experience out there. It's certainly not more features or better specs that attract people. It's not even the ecosystem since most consumers still use Windows PCs. So if only the latest iOS hardware runs buttery smooth with the latest software, why do people keep buying iOS devices and why to they maintain their reputation for excellent performance even as new iOS releases exhibit incredibly high adoption rates?
I'm not saying that older devices may not start to struggle with newer OSs, but that can't really be helped. But overall, I think that you get more out of your device if you don't deprive yourself of the latest software advances. Otherwise it seems to me like having a nice pair of shoes that you never wear outside because you want them to always remain in the same condition.
