No offense, but how the hell does a mobile phone need 256GB of storage?
Honestly, unless you're a freelance photographer on a budget that can't afford real photography equipment, I see no reason for this.
And, a 256GB iPhone 7+ would cost well over US $1,000. And that would be beyond ridiculous.
I think 4K footage takes up a lot of space (we are talking over 300mb a minute) then there's Live Photos, Apple Music downloads and apps. I think Apple know this and that's why they have done it on the iPad Pro. Yes it probably will cost around $1000 but if people are willing to pay it, plus people have been wanting more storage in the iPhone for a while.