I agree about Koby, but one can argue Oda designed King to be discardable, partly as a plot device (the Lunarians) and partly as a contrast to Zoro. Because King was completely subservient to Kaido, even if Kaido deviated from his idea of Joyboy, whereas Zoro also has his own dream and would probably not support Luffy, if he disagreed with Luffy's way or goal.