We have no confirmation that magic exists in the same way in SB as it did in FTL. The hat from Hat Trick in the only magic item that worked in a similiar fashion, even so until we have a follow up episode whether it was a success or not. Point being killing Gold with the knife may just kill him. If magic was the same in FTL and SB then August would have been able to control Gold. Rumple’s magic can’t be passed to Bae. Rumple got by killing Zoso. Blue Fairy also detected no magic in him but there existed a darkness. So if Emma picked up the dagger it is unsure what would happen, not to say it isn’t an important item for her to be in possession of.