While all of this is true, iPhones still managed to improve battery life. My iPhone Xʀ on iOS 12 is better than an iPhone 6+ on iOS 8, even though iOS 12 does a lot more and has a lot more features than iOS 8. Also more powerful CPU, GPU, etc, also more features.
If there has been a regression with the 14 and 15 series, then that’s bad, regardless of all that. The iPhone 13 on iOS 15, for instance, is ridiculously good.
I think this is just a plain and simple regression.
The iPhone 13 Pro Max on iOS 15 also does way more with more features and heavier usage than the iPhone 6 Plus on iOS 8. It is still a LOT better. It’s ridiculously good.
This is sheer software inefficiency, I think.