Lots of people. There’s a reason folding phones have been a big story for years.
Maybe they only sell to 5% of users but they sell and at top prices, meaning more possible profit.
And you can’t grow the iPhone at the current sizes. Everyone who once wants at one of those sizes already has one. You either have to convince them to upgrade or switch, and those are both hard.
However, if you give them a choice of something they haven’t had before, it’s way easier. Get the people who wanted to fold and went to android and would like to come back. Get the people who have a max and want something bigger. Get the people with an iPhone 13 who are simply curious enough to do the upgrade, but otherwise would’ve stood still.
Gotta sell.
I think they could get 0.1 to 1 percent increase in sales by fixing family/ screentime. It doesn't work flawlessly and a teenager is basically the same as a hacker and there are too many workarounds and it is very hard to harden either iPhone or Mac.
It also needs a kill switch. (Non parents will judge I am sure)
The device with good parental controls will be the go to for many parents regardless of practically anything else.