And this research is based entirely on Google searches for the terms "IPhone slow", dude you should really check your sources...
The main reason devices seem "slower" over time is because despite programs/"apps" being backward compatible to older devices most of the time, they are optimized for current hardware.
In other words, the apps move on but the hardware in the phone doesnt...
Plus alot of apps are built on huge open source libraries (because most developers are hopelessly fucking useless) and as such are disgustingly bloated.
But as an afterthought... if someone buys Apple products they obviously like being fucked in the ass anyway, so who cares?