People, except for a few imo, don’t really care. It’s like “how many angels can fit on the head of a pin” and people waste more time arguing about that than they’ll ever get back by staying on the original iOS release.
Most people charge when needed. Unless there is some drastic issue with battery drain or UI glitches people don’t care if they can only watch YouTube videos for 9 hours vs 10 hours.
The extremists, who never update due to supposed battery are far and few between and lose more functionality and time than gains in battery life.
Li-ion degrades due to calendar aging, can’t stop it. Historically looking forward most modern iPhones won’t have an issue with the next 3 or 4 years of updates.
You can’t prove the hypothesis: “for 100% of iPhone users upgrading to the next version decreases battery life by x%”. The best you can do is state for some iOS upgrades there have been reported decreases in battery life and it’s inconsistent across the board.
Most people want their phones to last a day. As long as their phones last a day with similar usage day by day, it’s all good. Apple also tends to improve battery life with the point releases.
I will grant that I care more than most, but I also think that expectations are different. As I said, the difference only ends up being massive (and by this I mean, enough so that it truly bothers most), after several major updates. It generally isn’t after the first major update, or the second one, perhaps barring redesigns like iOS 26, which has affected everything, including the 16 series.
Take the iPhone 13. The iPhone 13 is now four years old. iOS 18 is probably like-new on it. But iOS 26 isn’t. And users, instead of correctly blaming iOS 26, they blame phone age. They say “the iPhone is too old and therefore it isn’t iOS 26, it isn’t the device”, which is not true.
The few people I’ve seen online (of which there are more now, because at least some have learned) that stay behind, consistently report NO changes in battery life after years of running the same iOS version, even with reduced battery health. Which is what I have said all along. Batteries aren’t vulnerable to calendar aging if the device runs a good iOS version. As somebody who does not replace batteries, I would know.
The hypothesis you mention is irrelevant, because usage patterns are different. A heavy user will see higher drain, as the battery cannot cope with the massively increased voltage requirements. A lighter user like me will probably have better results. For me it may be a 20% drop. For a heavy user it might be twice that.
“Most people want their phones to last a day. As long as their phones last a day with similar usage day by day, it’s all good.“
And if you go back years on my comments, you will see that I have repeatedly stated that the solution was to increase initial battery life so much that the effects aren’t felt as much. I think that we are there now.
Now, a device with a large battery can take a LOT of garbage by iOS updates whilst remaining usable, even if worse vs the original iOS version.
Pro Max and Plus users since the 11 Pro Max can happily upgrade to iOS 18. This is the first time I’ve ever said this, but updating all iPhones from iOS 13-17 to iOS 18 was probably worth it. Even if there’s a difference it isn’t that high. I’ve tested this myself, iOS 18 is good on the iPhone 11, if you can tolerate some slowness. There is some lag. Lag which isn’t there on iOS 13, 14, or 15. But it is tolerable, and, imo, worth the trade-off vs the massive compatibility issues that would now arise on iOS 13. In spite of that, this lag is inexcusable. The difference is significant, but tolerable, even for me. I’m not the gold standard, but I have different expectations because I literally don’t know what it’s like to use a device that’s not performance-perfect as a main device since 2013.
But iOS 26 is garbage, and Apple should allow us to downgrade from garbage malware like iOS 26. They lost all of the good grace they had accumulated since iOS 12 with this. This is unacceptable.