Wouldn’t your liver run out of glucagon at some point?
“During a prolonged fast or vigorous exercise, glycogen stores become depleted, and glucose must be synthesized de novo in order to maintain blood glucose levels. Gluconeogenesis is the pathway by which glucose is formed from glycerol, lactate, pyruvate, and glucogenic amino acids.“
As long as you have fat, the body will convert it to glucose as needed, and as long as you are fat-adapted most cells will be able to use ketones leaving the newly created glucose for the organs that need it the most.