Because if it is possible we could help reverse the damage caused by the taken and oryx left behind and who know if the kell of the house of stone was taken we could save the houses that were taken by oryx but I don’t think something like untaken something has ever happened or maybe never will
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Theoretically Riven could be saved as she allowed herself to be taken but she retains enough free will that she can still function independently. So we could wish for her to be free and it would release her from being taken as well as break the curse.