For me, I think the ideal screen size is around 4.3". But good luck finding a high-end phone with that screen size.
Its also conceivable that whatever Apple had come up with would be bought not because the consumers wanted it (i.e. big screens) but simply because they wanted to have the latest Apple "thing."
In other words, bigger screens are simply trendy, and are therefore not here to stay.
Phablets may not be for everyone, but neither are the small phones that are rapidly becoming an endangered species.
Do the majority of smartphone users have large hands? I have fairly average-sized hands, and my Z3C is nearing the limit of what I can comfortably use one-handed.
Or it's not and your 'seems','maybe','would' and conditionals are just that :).