But that’s not how iCloud storage works. Or any other consumer cloud storage. You have a copy in the cloud whether or not a copy is on the computer, and unlike commercial clouds do not pay for data transfer.

I'm not sure what's your point. Apple makes local storage as expensive as possible/acceptable so that you buy more icloud storage for your files, photos etc and "stream" your content from the cloud instead to keep it stored locally. It's not secret that they see the "services" as being the biggest growth factor.

That's not how the 'iPhone full' message describes it.