So if you want the copyright of a work to expire, you need to arrange for the death of the sole copyright holder

That's not how copyright works (at least not in the US). when a corporation creates a copyrighted work (by way of paying the person(s) that actually made it), the duration is set as 120 years after creation or 95 years after publication. The lifetime of any employee is not taken into account. When a copyright is made by a person, it lasts until 70 years after that person dies. You cannot swap out that person for someone else, even if the owner of the copyright changes.

You are probably thinking of a method that is used to make private agreements last basically forever. A private contract technically isn't allowed to last forever, there has to be some point of expiration. To make a contract last forever anyway, they pick some condition that probably won't happen for a ridiculous amount of time, such as when the last descendant of the king of England dies (I assume they use this because the royal family keeps good genealogy records). If a currently living person is required, they might pick some infant relative to make it last as long as possible.