If I happened to buy 2 100% identical iPhones (for sake of argument) and use them for 5 years identically, but one I always updated the iOS software and the other I did not - I would expect less run time on the phone that has the latest software. But even after 5 years of perfect handling, the battery should be noticeably degraded. We pay a big price for thin batteries. The difference between the two phones, I might put at a 10-15% difference (educated guess). I imagine your prediction would be a higher %?
My point was:
Apple's own lithium ion batteries (lithium-ion polymer for the iPhone) - are guaranteed to hold 80% capacity for 500 cycles or 1 years (2 years if AppleCare+).
https://www.apple.com/batteries/service-and-recycling/
Lithium Iron Phosphate Batteries can do 10,000 cycles and maintain 80%+ capacity and are not harmed by storing at 100% charge for long periods of time.
https://www.howtogeek.com/822326/what-are-lifepo4-batteries/ - Hence the 10 year lifespan for these things.