Let's say 6 hour battery life was advertised as a feature. Now your phone after 2 years of use only gets 4 hours of battery life. They changed the phone in an update that brought it back to 6 hours of battery life by sacrificing speed. You can't have the best of both worlds.
Nobody is asking for the best of both worlds, but when upstream makes a decision as to how they want to prioritize one feature over another that changes how it currently functions, then some notification at a minimum is needed.
Batteries degrade over time. That's expected. Processors don't process fewer instructions over time though, that's someone making a decision, rather than letting the natural laws of physics play out.
Is that really such a foreign concept? You've been sold something with promises, and those promises are subject to natural occurrences and common conceptions, which are that some things will physically degrade without any outside action. Outside action to change that but at the expense of something that would not normally degrade, when it's about something that was marketed for sale, should not happen without notification and in most cases the ability to opt out.
Don't change how sold features are handled after sale if it flies in the face of assumptions without taking care about letting people know and opt out. It's not really that complex. All it requires is actually respecting your customers.
