There's actually two kinds of hierarchies (as defined by Graeber). There's the authority based hierarchies that anarchists argue against, and 'self-resolving' hierarchies, like Parent-Child, Teacher-Student, or Doctor-Patient that are inevitable and unproblematic. The difference is that the role of the parent is to raise the child, once the child is an adult the parent need not raise them more. They have become equals, and the hierarchy is dissolved. The student becomes educated and outgrows the teacher, the patient becomes well. These self-resolving hierarchies are fundamentally different than those based on continued authority and domination.