The issue is that Apple slows the phone AFTER you have bought it, and without adequate warning prior to purchase that this phenomenon will occur, whereas competitor phones have no such issue - the performance does not need to be throttled due to aging batteries.
Say all you want about this being "laws of physics" etc... its still bait and switch. Show me where on the box, Apple Retail Stores, or even on Apple's product pages where you buy the phones where Apple clearly states that the performance is going to degrade.
As of right now, if I go to Apple's iPhone 8 product page, it says nothing about the fact that it will slow over time:
https://www.apple.com/iphone-8/specs/. It goes through pains to tell you how the installed software will take up space on your phone, and to what level of water resistance the phone is, but nothing about the phone slowing over time.
Apple is going to see the lawsuits ramp up - and Apple will loose.