And how often will you put a usb cable in and out the camera during the years you will have it? Good luck with it!how do you know? it might charge faster than the ac adapter. USB-c is fast. I can charge my iPhone as fast as I can with a 5w adapter directly from my Mac book pro. Nobody can tell how things are going to be. it's just assumptions now. we will all have to wait and see real world users testing things out.
Like I said... IF, you can download and charge at the same time, it is a winner. If it takes 2 hours to charge, then it won't. Also remember the vertical grip should have 2 batteries. It would be great to charge both at the same time over USB-C without even removing the batteries from the camera.