When it dies or becomes obsolete (~5 years)
I think it’s a tech fanboys rather than Apple fanboys that change it every year : one can observe the same modus operandi for Samsung.
Most Apple users will keep their device for a while, just as most android users. Those that change tend to value peer recognition above eveything else and having the latest gadget is a way to achieve that goal.
Ah, the forbidden salt. Yes, I remember it fondly from when I was but a young boy, playing the GameBoy Color.