Just think of smile users as devil fruit users that ate the ability to laugh. You can't cure Luffy by removing his devil fruit power, that's why I was never on board of that idea, at best only someone with the expertise and technology related to devil fruits but that's it, and even then I find it a reach, if they can "cure" smiles then in theory you can "cure" any devil fruit user, just because it's tragic doesn't mean it can be "cured" magically.