Do they have a "responsibility" to do so? No. Should they find a replacement when they burn out / get bored / etc? Yes. I think so. But they are under no oblig

Do they have a "responsibility" to do so? No. Should they find a replacement when they burn out / get bored / etc? Yes. I think so. But they are under no obligation to do so. If it's popular enough, someone will pick up the reigns.