Again, most USB3 (save sticks) devices, such as external drives, printers etc., use a separate, detachable cable. All you need is exchange that cable with one that has a USB-C connector at end. Those are dirt cheap. For the majority of the users, this won't be a problem. Problematic are external mice/keyboards and USB keys.
Right now, when I come to the office, I need to plug 5 cables into my MBP: network, audio, power, HDMI, backup disk. Should Apple switch to USB-C completely, I will only need to plug in a single cable - the one to a hub. Seems like less of a hassle to me