This marketing tecneque has been used for many years by many differnet companies. The idea is genius and great for profit

(bad to the customers however

)
Think about it this way. You want to make money. You build up a reputation so good, it is hard to break. Then, you deliver a new product that everyone expects to be just as good as its predecessor. However, the company made more of a profit for selling the dummied down item than the high tech one (or even one that breaks the predecessor's durability quicker)!
General Motors used to do this
Dodge/chrysler is guilty of this
Even Honda is guilty of this
Looking back at your iPhone, (same model different iOS) you have the same phone. However, the software slows your model down and, causes it to glitch out more. By doing this, the consumers will go out earlier and buy there new iPhone once there contract is up.
How about durability? Accidently drop a iPhone 3GS, your fine wit just a minor scratch. Same drop, same conditions with a iPhone 6+, oops your phone has bent, it now has a black blob in the screen from the shattered LCD and, oops it's totaled

looks like you have to go out and buy a new iPhone 6+!