Hello Brian,
I dont't aggree with you concerning that a battery after 2 or 3 years of use is worn out and has to be replaced. My iPhone 6 is years old and with iOS 10.3.3 battery still was great with it's 1550mAh.
Concerning the drain problem I wrote in some other threads:
For me it's not a problem of draining but a problem of charging the battery.
With iOS 10.3.3 the battery of my iPhone 6 at 100% of charging had a capacity of 1550mAh (reading of app "battery life"). After upgrade to 11.01 "battery life" tells me 1000mAh, after upgrading to iOS 11.02. 1400mAh; same after upgrading to 11.0.3.
I checked the capacity always after 100% charging and the capacity of the same battery in the same iPhone differs within hours so much before and after the upgrade. I can't believe that it's fact of normal wear.
I believe the charging process stops before the battery has it's real 100%. And that's software-/ iOS-related.
My conclusion is that, when the battery charging process stops before the battery is fully charged but tells me 100%, it seems to drain fast because it never was charged to it's full capacity.