I'd rather have a "good ending" and be able to take one of the girls to the prom before starting the cycle of endlessly adding new fuckable characters. I'd settle for finishing the routes for the existing characters before starting the cycle of endlessly adding new fuckable characters.
I don't think there's actually a perfect way to do it. Having very few characters certainly makes it easier to focus, but in return the "world/game" is a lot less divers and probably also a bit less interesting. Don't get me wrong, there are many games which get it right with few characters, but in SS's case I'd say that girls like Missy and Becca or Grace and Odette actually make the game better, even though they certainly aren't necessary to write a good story about Roxxy or Eve. The thing is that you have to include them into these updates or you may not add them at all.
I know the main story has kinda messed with people minds, but apart from a few characters, there aren't really that many "super optinonal" characters people would forgo easily. Unless you get radical and cut everybody except the 8 (9) official main LI. But then the game would basically only consist of Debbie, Jenny, Mia, Roxxy, Eve, Ronda, Annie, Judith and Kevin (maybe Diane and Tammy as it would be easy to add them).
Last but not least to your point: The prom is supposed to be added after we've gotten a somewhat comprehensive dating system. That's why DC probably never even considered going beyond what we've got now, for now.