I agree with you about iOS bloat and such. I don't think we disagree overall. My point was more that a device that lasts 40 hours with 100% Battery Health typically doesn't last close to 32 hours when it's battery gets to 80% health.
Absolutely, if it takes you long enough to make battery health drop to 80%, and in the meantime you accumulate enough iOS versions, then the difference will be far higher. Which is why many people dispute this: they see that they use a device and update it. Battery health degrades, they replace the battery and it improves. So they say that battery health is the only relevant aspect, because that’s all they see: they never try staying behind for years.
To your point, the Battery Health metric may be more linear on devices running their original OS and the metric may become less linear as newer versions of iOS require more minimum voltage from the battery to keep the same process chugging along doing the same things. I don't have the controlled means to test that but it makes sense to me.
The metric being unreliable is something I haven’t experienced both with updated and original-version devices, but then again, I truly don’t have enough experience with the iOS 11.3-and-onwards in-built metric to tell. I can confirm that the minimum voltage to run newer versions is far higher, which is what impacts battery life in the first place.
Practically speaking once we upgrade iOS, we can't go back, and most people can't stay on old versions of iOS indefinitely.
My perfect phone however would be an iPhone SE (2016) running iOS 12 (and for all I know, I would be just as happy on iOS 9 or 10 as I don't really remember the differences) but updated/patched Safari. And if we're wishing, throw in a USB-C port...
I agree wholeheartedly with this. My perfect iPhone would be an iPhone SE (1st-gen) running iOS 9 or 10. Of course, like I said, I have an iPhone 6s running iOS 10, and I had to stop using it (much to my discontent and disappointment!!) because it wasn’t compatible enough anymore.
But let’s create a hypothetical world in which everything is magically compatible forever (like you said, including Safari, which I can confirm is practically unusable today) and the small SE would be a dream, barring…
OP said that they are using the 13 Mini. That iPhone has a great camera. A dual camera, with a full suite of features, even if the quality doesn’t match my 16 Plus (full portrait mode, the Ultra Wide, OIS, Night Mode, etc). The SE’s camera is too underwhelming nowadays for me. Battery life would be quite meh too (it was between the 6s and the 6s Plus on iOS 9 for a light screen-on time of about 9-10 hours. For reference, my 16 Plus gives me 27. Yes, twenty-seven hours of SOT). Too much of a difference. The larger screen is something I’ve gotten used to and deliberately pursued having smaller options.
I’ve abandoned smaller iPhones and adapted to the larger chassis even though it is slightly more uncomfortable.