If Oni takes an update or two to add new quality of life improvements or content to the existing girls, like new clothes, a new location, new activities or things like that, he will have to do the same thing to all future girls, which will increase update times and possibly piss off some patrons, both those that want more updates and those that want new girls or more content.
After all, adding a new girl is the best way to create a substantial amount of content and make updates look "big", you have to do all her events, see her conversations and her reactions, fuck her, etc. Meanwhile, adding content to existing girls or something new will make the update look small, because if you have them all in 1000 in all stats, seeing the new content is easy.