Of course, the 15 Plus’ battery life was just too good.
Was it? Outside of better 'spec sheet checklist' bragging rights did it lead to higher sales? The Plus has had similar issues that the Mini had in competing in a crowded iPhone line up. Apple appears to have thrown "max battery life" at the plus (e.g., 100 hours audio playback versus 95/75 for the Pro Max and Pro respectively. Also substantively better all-around for the 'plain' 15. ).
If the "more battery life" feature didn't substantively boost sales , then a pretty good chance Apple will just take better margins ( or probably less likely price reduction) on a slower unit moving phone. They would just settle for this phone being "bigger than a plain 16" and just leave it at that for differentiation. If that leads to roughly equal sales then it is a better fit with the rest of the line up. Some folks they will 'walk up' the product line to Pro version and others will fall into the plain iPhone.
It is a relatively weak differentiating feature when plain 15 users can just buy a MagSafe battery pack if maximum battery life was really the absolute top priority. $50 versus Apple's $100 for better battery life. That should be pretty competitive. Not sure why Apple would 'bet the farm' that would work on customers hyper focused on that narrow aspect.
It would also be a better fit when the 16 becomes the 'n-1' iPhone sold during the second year. If the iPhone 15 Plus didn't ramp sales higher it is going harder on the margins to sell when the volume goes down versus the iPhone 16 Plus over next sales cycle. That bigger, more expensive battery gets tougher in year 2 if it isn't offset by economies of scale benefits (over both years). If Apple sells the Plus in the 'n-2' slot on the 3rd year cycle... even more so. ( that would be longer than the 'Mini' iterated. )
P.S. Note the substantive increase in the 'plain' 16 battery size. It is going to have a bigger screen (which will incrementally 'eat' into Plus sales) and perhaps also longer battery life than 14/13/12/11 plain iPhone (if battery increase is relatively bigger than screen size increase power consumption) ... which again 'eats' into the battery life feature as a differentiator. ). When Apple makes the plain iPhone 'better' then the Plus gets squeezed from both sides ( 'plain' and Pro line up) even harder.