They all have the same apps that 90% of people use (Facebook etc), they all have the same core features that 90% of people use (texting, internet and calling) and yet.. many people upgrade often.
I expect its just a newer = better mentality that humans share.
(There's also the fact that for many, the upgrade is effectively "free" with their contract.)
My use case is perhaps in the minority (I used my iPad primarily to read, watch movies via Air Video, browse the web, keep up to date with friends via social media, bedtime email etc), but still did it 5.5 years strong.
The system auto-updates your apps, whether you want to or not. Eventually, the newer apps simply refuse to run on older versions of the OS, and you can't get OS updates for that phone anymore.
The result is that to the extent that you care about using any apps, you have to buy a new phone, even though your old phone's hardware is still more than adequate in a technical sense.
They had stopped releasing new OSes for the 3GS some time ago. The Uber app had been auto-upgraded and now simply announced that it refused to run on such an old OS.
I was left standing there without transportation.
The Lumia 920 was released in 2012, and has an 8.7 megapixel camera with an f/2.0 Carl Zeiss lens, and supports 1080p video capture. You are not limited by the camera.