Yes, I’ve been saying this all along. Having said that my comments about the a12 still stand. My own anecdotal opinion is that my Xs max with a new battery on iOS 17 performed as miserably as in the year I bought it.
Perhaps you are a very heavy user in terms of apps used?
Like you said, the reason for which iOS updates decrease battery life is due to increased power consumption. What’s the problem? Very simple. If you are an efficient user, with light to moderate use, original iOS versions are amazing, due to their efficiency. New iOS versions obliterate this, because they’re inefficient and have too much power consumption. This is why, for example, the iPhone 6s has paltry battery life even with light usage on iOS 15.
Now, even if the device isn’t updated, heavy use has always resulted in poor battery life. If the usage is heavy enough, battery life is obviously more abhorrent when updated, but the user won’t recall the original version having good battery life regardless.
Allow me to give an example: 1st-gen iPad Pros, iOS 9. Some people used it for drawing with the Apple Pencil. They said they used it at max brightness for various reasons, and reported battery life struggled to get to 4 hours. With that usage on iPadOS 16, you’d get one hour, but the user won’t recall iOS 9 as being particularly great anyway.
If power consumption is high enough, original versions won’t give a good battery life anyway.
You frequently mentioned “but what if you use it at full brightness on LTE for GPS?”, and no device will give a good battery life with that usage. Will an iPhone Xʀ on iOS 12 be infinitely better than one on iOS 17? Yes, but you can probably drain one on iOS 12 in... what? Four hours? Not great.
That said, there’s no iPhone that will give you good battery life with that usage.