Royal Icing is "food." Treat it with respectful food-handling techniques. Sometimes people think because it is hard, it should last like cement forgetting that the cookie itself looses crispness when set out in humid environments (with or without icing), the character of icing changes, and age takes its tole. If you wouldn't set out chocolate in a non-air conditioned summer-time, don't expect your cookies of any type being peak freshness for the same reason. Note that I live in a high-humidity area which can "sog" any baked product quite quickly when the conditions are right.