Decaying battery is a serious issue in the long run. As long as you’re not buying a 5 year old phone, you should be fine. Get something that’s about 1-4 years old instead and check the battery health before the purchase.
If you need a battery replacement, that means that the previous owner abused the battery with suboptimal charging style or maybe the phone is old enough that you won’t be getting updates any more. In the latter case, it’s just pure planned obsolescence.
As for the article, slowing down the phone without the users consent was a bad move IMO. This shows how little Apple respects the user. Then again, tech companies tend to treat us like trash, so that’s nothing new.