You imply that the thinner devices came with a shorter battery life. Which you know isn't true. Yes, making the phones thinner meant that battery life was not improving as components got smaller and battery technology improved. But "not improving" is not the same as "getting worse".
Use the iPhone SE as a case study.
As for battery not getting worse, sure if you look at the Spec sheet on Apple.com, in real life, iOS updates would kill the battery in horrible manners.
I'm in the boat that recent iPhones have gone backwards in battery, cause iOS is the problem in all this.
The damn SE bucks the trend.... my next iPhone