This article states that Apple did that to increase battery life. Do you have other sources to back your claims?

They LITERALLY lost a court verdict that said they did it to screw customers.

I don't care what Apple's PR said, the court found that the AG complaint of "Many consumers decided that the only way to get improved performance was to purchase a newer-model iPhone from Apple, ... Apple, of course, fully understood such effects on sales." was correct and that Apple had to pay up.

Apple admitted slowing down old phones. They DID NOT admit that the reason they did so was to force people to upgrade sooner. That is something you made up.