Forcing large amounts of sugar into body tissues takes huge amounts of Insulin, even when exercising - so chronic use of sugars to fuel exercise can still lead to Insulin Resistance, and T2DM. There are plenty of middle-aged triathletes and runners out there with T2DM and metabolic syndrome.
Also - relying on a continual flow of carbohydrates becomes a problem on a long ride if suddenly you can't eat for some reason; gastric distress is pretty common on endurance events, and if you're reliant on a steady intake of food, you'll be in trouble.
When Insulin is elevated, burning stored fat is shut down completely - so once you've taken that step onto the carb roller coaster, you're stuck there for the rest of the event.