I agree. I think that Sidney’s days are numbers. I don’t see that his character adds anything to the show now that Emma knows that he is working for Regina. Regina won’t have any use for him anymore now that she can’t use him against Emma – he doesn’t serve any other real purpose in SB. In less, of course, they start to have the characters pick sides and once he discovers that Emma realizes what he has been up to and that she is gaining more power he decides to switch teams.
But I also have to say that after seeing the promo picture of the broken mirror, I wondered if that was symbolic of Regina killing Sidney. Kind of like how when she squeezed Graham's heart, he died. Maybe if she breaks one of her mirrors, Sidney will die.