But then you come up against the vigilance problem. People are remarkably bad at maintaining preparedness for rare events.
Like, in that decade between attacks, someone will realize that have a ton of bows that aren't being used except for target practice. They'll realize that they can maintain practice with, say, half those bows. And when one of those years is lean, and the king has a war coming on, selling off the bows starts looking like a good option. "We'll still have years to rebuild the stockpile!" they'll say. And "Dragons almost never attack," they'll note. And then the bows aren't there when the dragon does attack.
And, there's the 'over-specialized plan" issue. They do all this preparation for dragons, and have no effective defenses when the bulettes come...