A "property right" is an entitlement to government protection for that property. Society is willing to protect Manhattan indefinitely - it only takes a fixed number of police officers to do so. But protecting everyone's unlimited imagination indefinitely is impractical (as time went on the number of historical creative works increases, and people continue to own all the rights to them, so we get an ever-increasing number of lawsuits per new creative work), so we set some time-based cutoff (which is an imperfect approach, but we need some rule).