The obligations of contracts can and sometimes do survive the disappearance of underlying IP rights. If someone agreed to pay Warner 10$ for the right to sing Happy Birthday each time their movie airs on TV, that contract is likely still in effect. Consideration (what one party gives in a contract) is measured at the point of formation, not years later. An agreement to not to sue someone is not premised on the validity of that theoretical lawsuit. An agreement to not attempt something is still an agreement even if said attempt would prove futile. There are cases where a contracting party has been forced to shell out for something anyone else could use for free. So if anyone reading this has signed a licensing agreement, talk to your lawyer before ripping it up.