I think the real solution here is to better empower potential parents to have more freedom to make the choice of whether to become parents or not. If someone doesn't want to become a parent then I think it's a good idea to remove any obstacles between them and avoiding being a parent.
Too often that choice is forced on folks and the major impact that has on your life can leave people resentful and angry which just isn't a good place to start being a parent from.
If parents are stone/ drunk so regularly they shouldn't parent, it's child neglect/ abuse regardless of whether the drugs are legal.