Eat too little carbohydrates, and you'll die. Eat too much, and you'll die. Your claim that any increase must lead to the same marginal results is absurd.
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.
Eat too little carbohydrates, and you'll die. Eat too much, and you'll die. Your claim that any increase must lead to the same marginal results is absurd.