I'm still enjoying my 3-year-old non-iPhone, and intend to keep using it until it stops working entirely, whereas my spouse with an iPhone was already voicing suspicion before this announcement that Apple was intentionally slowing their old iPhone through updates, just to provoke an upgrade. With the announcement, I can't fault the reasoning.
How much of the used market for iPhones are devices that had been traded in (because slowed by Apple due to old battery), refurbished by the mobile phone company (probably just to replace the battery and reset it to factory settings), and then resold to people whose older model refurbished iPhone now needs yet another new battery? That generates an additional sale of an entire old iPhone, whereas other phones would only generate a sale of a replacement battery at that time.
I think you would probably have to look at what mobile phone customers are actually using, right now.