iPhone 5s with A7 chip from 2013 is getting iOS 11. iPhone 5 is ending support because it was the last with ARMv7 (32-bit) processor and Apple is going all in with ARMv8 (64-bit) compatible chips.
So, i believe that Apple will support iPhones for at least 4-5 yrs.
[doublepost=1497108721][/doublepost]Remember, Apple is so far ahead of everyone else (mobile) in chip processing. Even though Android claim that they can do "more" with their OS, their latest chip CPU processing is around Apple A8-level. Qualcomm S821 (late 2016) is slightly below A8 (2014) and their newest S835 (2017) is still below A9 level according to Geekbench single thread.
For example, Pixel (late 2016 with S821) single thread score is BARELY better (around 200 better, which is almost insignificant) than iPhone 5s A7 chip from 2013!!
So, if Android with all their fancy do "more" claims and they are literally at least 2 years BEHIND Apple chips, then i think that Apple iPhones have a very long shelf life.
(Yes, i put quotes around "MORE" for a reason....)