Maybe if Apple refused to support new features on old hardware it would solve this issue, but then you just anger users who are forced to buy new hardware to get a new feature...
Every major iOS update adds more functionality that
consumes more CPU cycles even in the background.
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.