Do you mean that you should be able to keep Sage and Bella options open when you choose to be single, or do you mean that you should be able to keep seeing Sage and Bella options open if you choose another girl? If you mean the first option then I agree about Sage as she was happy with just being fuckbuddies and the problem comes with DPCs pacing of the relationships, Jill and the throuple were in the right place for that decision but Bella and Sage weren't. However I don't think Bella was that far away from being there and wouldn't have put up with being just one of many girls for much longer. If you mean the second option I disagree totally as choosing one girl means you've picked the girl and you shouldn't be hooking up with any other girl from that point on.
Well, I could agree with you for Sage as a fuckbuddy, maybe Bella too, but we should also remember and take into account what Elena says to the MC just before the big choice at the end of episode 8, where "if it didn't clicked yet after going through the end base, it's probably not meant to be", where the MC own wishes is to clear his head on where he's at on his love affairs, the MC respects both Sage and Bella, he cares about them, and doesn't want to just "use" them, he has the opportunity to chose Sage or Bella if their path are still active, so in the case you chose Others while Sage and/or Bella were options, it does mean you're not chosing them, so yeah, prefering to end it with them kind of makes sense since in that case, as Elena advised, it would only make things worse with time. But yeah, maybe if you're a massive DIK, you don't give a fuck and keeping fucking them for the sake of fucking them without the feelings, could make sense, but with the "care about them" into account.... For the Sage path at least, the game does show you the door isn't completely closed in the future