> How about slowing down the OS when the device gets older to “save battery life”?

This is false, it was to avoid situations where the battery couldn’t provide enough current and would likely cut out causing the phone to switch off. It seems the intentions were reasonable to keep older phones usable for longer and they fairly quickly rolled out a large discount on the cost of battery replacement when the public outrage started.

I strongly disagree. The intention may have been to avoid abrupt power downs but their implementation also made sure people upgraded their devices rather than buy a new battery because most people would never be able to imagine that a device could be slow because of a bad battery. This, IMO, was a dark pattern. A sudden power down would be clear indicator that something is wrong with the device while a gradual slow down of the device would seem like that the device is just getting older and needs to be upgraded to a newer one.

Ok, you have a phone that got slower, but works. And you got phone which just switches off randomly.

Which one are you more likely to replace sooner?

> you have a phone that got slower, but works.

I would assume that like most Android phones and (2000s) windows laptops the phone is just old because of age and I need to buy a new one to get faster one.

> And you got phone which just switches off randomly.

I would assume that something is wrong with the phone and would get it looked into at-least once to know what the issue is. Apple would (presumably[1]) tell me its just the battery which needs to change nothing else and I would do just that.

[1] I have read articles where it is mentioned that even geniuses were unaware of the throttling feature.

How about one that displays a message that the battery needs to be replaced?

I'd get the battery replaced and use it for another five years.