My father has uncontrolled diabetes. All of his extremities are severely affected as well as sight and brain functions. He has an extremely sad life but it was his choice. That's what too much sugar can do.
A guy like Michael Phelps would eat 20,000 calories a day when in training - mostly from carbs (sugar), but I don't think he ever had to worry about diabetes. In his case, sugar gave him the energy he needed to perform physically. Too much sugar is subjective, but sugar shouldn't be labeled as toxic.