If Apple slowed the phone down upfront then people would complain that Apple were throttling as the user is “too dumb to look after their battery properly” so instead they are doing when they can point to a proven need to throttle the device. This feels like a no win situation for Apple unless they had/will put in “throttle my phone so it doesn’t go weird” switch in settings.
Side note: we’ve known for years that hammering a battery day in day out causes problems, my wife and I have had our 6S phones for the same amount of time - I plug mine in as much as possible (over charging is not a problem anymore) and she runs hers into the red all the time. After 2 years my phone runs fine and, when needed, I get a days charge out of it but she barely gets 4 hours and is noticeably slower. If people looked after their batteries better this would be less of a problem.
Side note: we’ve known for years that hammering a battery day in day out causes problems, my wife and I have had our 6S phones for the same amount of time - I plug mine in as much as possible (over charging is not a problem anymore) and she runs hers into the red all the time. After 2 years my phone runs fine and, when needed, I get a days charge out of it but she barely gets 4 hours and is noticeably slower. If people looked after their batteries better this would be less of a problem.