I don't know squat about USB C (I don't have any equipment that uses it yet), but with A & B, my solution to this problem is simple -- I use charging-only cables that don't pass the data lines through.
USB C has a power delivery spec that permits increasing voltage above 5V; in order to do that you need to negotiate what each side (and the cable, too, I think) supports before increasing voltage. That's how laptops that use much more than 10 W can use USB C as their charging port.
But that's only for high voltage charging, right? You can still get 5V without negotiation, if I understand correctly. If so, that's a tradeoff I would be perfectly fine with.
USB-C is being used to power things where 5V is not remotely sufficient. So while for some devices (cell phones, largely) 5V is a plausible tradeoff, I don't really see a world where many MacBook / Chromebook / etc users opt for 5V max charging power.
Fair enough, but I was really just talking about safety when using public charging points, which implies smaller devices. Also, I was really talking about what I find acceptable. Other people's mileage may vary.
