I would agree that Sidney’s days are potentially numbered – especially now that Emma has figured out who he is working for. Just like Graham, I don’t think Regina has any use for Sidney anymore now that he won’t be able to use Emma to bring info back to Regina. And we’ve seen what Regina does to her “pets” when she no longer has any use for them.
Also, I believe at Paley Fest someone mentioned the actor who plays Sidney is on another pilot that they were hoping would be picked up. Not sure if it was picked up or not. Kitsis and Horowitz said it wouldn’t be a problem if it was because it would just be a matter of working out schedules but I just don’t see any point to his character any longer now that Emma has figured out what he has been up to.