or an USB-C to Lightning cable. Those exist. And so do any USB-C to any other USB-Plug cables.
Considering that Apple revises their hardware builds about every four years, it makes sense to move to strictly USB-C now as two years from now, I'm quite sure no more peripheral will come out that has anything but USB-C.
So within the next 2-3 years you'll have collected heaps of USB-C cables that will connect your devices directly to your MacBook Pro. No dongles what so ever.
USB-C is so much better than anything other USB that came before to the point where it can (and will) easily replace not only all other existing USB plugs out there, including on the device end (where there's a mess between USB-B, USB Mini and USB Micro right now) but also has the potential to replace the various ways how we currently connect screens (DisplayPort, DVI, HDMI, or even VGA).
Going purely USB-C means that they could optimise as much as possible for this new connector without thinking of wasting space for a useless connector three years down the road, especially as you can convert USB-C to any other USB, but if they added an USB-A port, that would not be useful for anything but USB-A plugs.