Hypercalcemia itself is reversible, but things it causes might not be. The most likely example would be renal stones, which can cause acute kidney injury and permanent renal damage. Paralytic ileus can lead to bowel perforation requiring major abdominal surgery. Hypercalcemia will make cardiac myocytes relatively refractory to depolarization, which in severe cases can lead to cardiac dysrhythmias and cardiac arrest. Hypercalcemia is not a benign condition.

Important to specify this only applies to type 2 diabetics. If a type 1 diabetic stops taking insulin, they will die.