You know what happens when insulin fails, don’t you?
Yes.
Here's what you said: Sugar is toxic to most cells. It’s only tolerated (in low concentrations) in most organisms because it’s such a cheap source of energy.
Sugar is not toxic to most cells. You use that word but I don't think it means what you think it means. To say sugar can only be tolerated in low concentrations in most organisms is ridiculous. Glucose is sugar and it is the only source of energy for your brain and cellular functions. It has nothing to do with being cheap.
Any athlete on a high carb diet will show how wrong you are. As I said before: through glycolysis, the citric acid cycle and oxidative phosphorylation, glucose is oxidized to yield CO2 and H2O which yields energy in the form of ATP. No ATP = No energy.