I'm a huge fan of Android, however I did switch over to an iPhone once for a few cycles. I noticed some things, then I noticed them again in the latest iPhone release. 

Everytime an updated iPhone comes out, it seems that an OTA (Over the Air) update also becomes available for old iPhones. The update never contains anything useful. For instance, when Siri first came out, you had to buy a new iPhone to get that. Never mind that all the voice recognition is done in the cloud, old phones supposedly "couldn't handle" the load.

For the most part, I would ignore major updates to the iPhone's OS. However, sometimes you have to do them otherwise your carrier magically can't support your phone anymore. Whenever I updated my phone, it would grind to a screching halt. The new OS was just too bulky for the phone to run.

My suspicion is that this is an Apple plot to get you to buy a new phone- update your old phone with software that slows it down so much you have to get yourself a new phone. 

Of course, I can't prove any of this. My examples are purely anecdotal. It just seems oddly suspicious that every time a new iPhone comes out, all the old iPhones grind to a halt.