[uniqueid]

  Every major iOS update adds more functionality that 
  consumes more CPU cycles even in the background. 

The very best OS updates also revise or replace existing code to run better (more reliably, more securely, more quickly, etc).

The larger issue, in any case, is that Apple prohibits reverting an iPhone to a prior version of iOS.

This inexorably leads users of old devices to one day install that final update that turns their device into a turkey.

At that point the customer buys a new phone, a year or two before they intended, and the old phone, which is too slow to be useful as a back-up device or hand-me-down, goes into a landfill.