For technical reasons, there can't be more than 100 characters (besides Yandere-chan) at school. Once all the clubs have been added to the game, there won't be any room for rivals unless he gets rid of some people. He decided that Kokona, Saki, and Riku should all stay, since he put so much work into those characters already. As for the others, Dev has other plans for them. He mentioned that he wants to turn them into a famous pop group, so we'll probably end up seeing them on posters and hearing people talk about them.