Most things decay over time, but, just like all other themes, decay can be understood in a multitude of ways. There's physical decay, but there's also moral decay—both often go hand-in-hand with old organizations, and physical decay can be used as a metaphor for moral decay. And decay isn't always a bad thing - an evil empire might crumble so something better can flourish. Think about how people in your world try to prevent decay too!