Think about it, how many people bought a new iPhone because their current model became too slow, not knowing that a simple battery replacement could have solved the issue? And it wasn't just a limited range of iphone 6s models.
https://www.theverge.com/2017/12/20/16800058/apple-iphone-slow-fix-battery-life-capacity
"Some Reddit users report that replacing their batteries has returned performance and CPU clock speeds back to normal. The reports are particularly troubling because any perceived slowdowns by iPhone users might tempt owners to upgrade their entire device instead of replace the battery. “This fix will also cause users to think, 'my phone is slow so I should replace it' not, 'my phone is slow so I should replace its battery,’” says Geekbench’s John Poole."
Geekbench developer John Poole has mapped out performance for the iPhone 6S and iPhone 7 over time, and has come to the conclusion that Apple’s iOS 10.2.1 and 11.2.0 updates introduce this throttling for different devices. iOS 10.2.1 is particularly relevant, as this update was
designed to reduce random shutdown issues for the iPhone 6 and iPhone 6S. Apple’s fix appears to be throttling the CPU to prevent the phone from randomly shutting down. Geekbench reports that iOS 11.2.0 introduces similar throttling for iPhone 7 units with older batteries."
"It’s also clear that Apple, which
makes its devices hard to open and repair, could do a better job helping consumers understand the benefits of battery replacement. That’s something the company seems less inclined to do when it might mean forgoing the sale of a new iPhone every 12 to 24 months."