Ya, this is such an annoying facet of these types of games and you see it with nearly every game here. Devs are always adding in unnecessary side characters and the only reason they're doing it is to do like you said, fill in space and make the game seem longer/bigger and delay content with main characters. They can boast about how many scenes, renders, animations there are while at the same time basically throttling content with the main characters. It takes longer to get to the content people actually want and to develop the game, and devs love that because it means more money for them.
I wish more devs would stop throwing in unnecessary side characters to delay their game, just finish the game and then you can start a new one with new characters. I think devs would gain more support and make more money taking that approach anyway, but maybe they don't want to attempt that because they're not confident they could make another successful game and don't want to take the risk and would rather throw in unnecessary characters to the game they're working on now so it takes longer to finish.
