Typically, when consumers tire of a product, or it no longer meets their needs, they do look elsewhere. Just take a look at all the once great brand icons that today are considered mediocre at best, out of biz at worst.
That's often true for single purpose products, however smartphones are a mixture of multiple products and features.
Smartphones are kind of like those office all-in-one printer/fax/copiers, where we often have to decide which features are most important to us, and sacrifice on what's of lesser importance.
I.e. screen size is just one of many factors most smartphone buyers take into consideration. For people who are invested (mentally or otherwise) in iOS, the single attribute of screen size probably is not enough to make them switch to a whole other ecosystem... whether it goes bigger or not.
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The upshot is that we cannot know which screen size the majority of current iPhone users prefer, until/unless they have a choice within the iPhone world.
From decades of experience, however, I can totally guarantee this: if the iPhone does go to a larger screen, then even the majority those iPhone users who don't want it now, will eventually forget and come to think of the new size as the new "normal", and then will not want to change from THAT size
That is just human nature.
Yes, I see your point. Of course, if the phone is larger to begin with, and I use a case with that it becomes larger still.
lol ... Touché !