If brands see that increasing the amount also increase sales without increasing the risk - they're likely to do that - which could numb the sweetness of anything else.
Given how sensitive consumers are to changes in its taste, I think the overriding concern for Coke would be not changing the flavor of the formula.
I'm not sure anyone's clamoring for sweeter drinks. Non-diet drinks like Coke have an established recipe, and diet sodas approximate the same balance of flavors.
Anecdotally, even as a kid, I found Skittles to be unpleasantly sweet. To this day, I avoid most sweet soda drinks and seldom drink even those I can tolerate.
It is not discipline for me. It is not even taste. It is tolerance.