I also wouldn't be surprised if the PCs win. Dragons are extremely reliant on their breath, and can get outclassed if it's not properly used. As an example just this past weekend for some fun I ran a scenario where a couple of friends each controlled a Young Dragon (Blue and Silver) against a village of Hobgolins and Orcs and a Chain Devil leader ~32 opponents in total, half just being the CR 1/2 Hobgolins and Orcs, with a mix of leadership from Half-Ogres up to a Hobgoblin Warlord. The Dragons ultimately got shredded once they stood and right instead of using hit and runs with cover. Attacks add up and Dragons aren't super resilient.