They could have a lean version of the macOS desktop on every iPhone where it would switch to the macOS desktop once you plug it into a display. Include iWork on both sides with shared storage and you could switch between the phone interface and macOS.

I mean Samsung DeX has done something similar, but Apple could make a much more powerful version, since they have a real fully-developed desktop and they have apps for both platforms that use a single code base.

They will, of course, never do this, because it would result in losing Mac sales. Though I think less than one would initially think, because a laptop is much handier on the go than carrying a separate display, keyboard, and mouse and the lapdocks or whatever they are called have much worse displays and keyboards.