A generally astute assessment of toxic relationships and how to recognize them. The only thing I would add is this: if the toxic relationship is a marriage, a sibling relationship, or a parent-child relationship, we should make every possible effort to resolve the conflict before giving up on it—even if this entails couples counseling. The reason is that the breakdown of such intimate relationships can be deeply traumatic to one or both partners and injurious to children of that marriage in ways that may never heal. Far too many people choose divorce for entirely selfish reasons, rather than even trying to heal a troubled relationship.
As for less intimate relationships, however, your advice is sound: if it is one-sided, parasitic, or stressful, let it go.