It was a measure to increase product longevity (e.g a slow phone was seen as better than a phone that shut off randomly when voltage dropped)
If this was such an altruistic measure why would the geniuses at the Genius Bar deny that my mothers phone had slowed down and throw diagnostic tests at our faces? Why was the exasperated Genius, who could clearly tell that the phone was slow indeed but his successful diagnostics meant there was nothing he can do, reduced to swiping up all the apps on the phone to kill them, when we both knew that if iOS was working properly that should do nothing?
My iPhone 5 got so slow and Apple was so unhelpful that I ended up replacing it. That was my first and last iPhone.
But even without Apple, that particular battery was pretty easy to replace
Also: users were not warned about this "feature", many (most?) didn't know why their phones slowed down, and there was no way to disable the behaviour.
And I already mentioned in my initial comment that they didn't notify users. That's what the lawsuit in France was over, the lack of notice. I believe an OS update did eventually let users switch off the behaviour, and view their battery health.