Not all phones support all the alternate modes of USB-C, especially display port or HDMI, so display output is often a hurdle.
Good point. I guess few people use those since phone OSes don't scale well for desktop use.
The latest MacBook is essentially mobile hardware with desktop peripherals and software. Seems like the right way to go rather than bloating a smartphone with desktop software given that the amount of people who want their smartphone to be their only interface to everything is likely a very small minority.