Every long-running comic strip has a few characters who got phased out early on. The most famous example might be Lyman from Garfield, Jon's roommate and Odie's original owner, who simply disappeared one day around 1985. (Davis has occasionally joked that if you want to find Lyman, you should dig up Jon's basement.) You could also point to most of the early human characters in Bloom County, Nicole in Doonesbury getting sidelined in favor of Joanie Caucus, or the abortive Uncle Max arc in Calvin and Hobbes.