Actually it's disputed wether carbs are needed at all. It might be possible to function on fat alone (for energy) https://en.m.wikipedia.org/wiki/Ketosis

This is not really the point of the thread and I know what you are trying to say but your example is bad. You don't need to eat any carbohydrates at all. Your body can manufacture enough glucose for your brain on its own.

I have to contest the "eat too little and you'll die" statement. Human's can survive (and thrive) on zero carbohydrate diets. Gluconeogensis will provide any glucose the body requires.