"A clone is an exact genetic replica. It is a genetically impossible for the quality/potency of the plant to degrade"

Not entirely true. Cloning is a method of analog reproduction, and as the strain continues it's possible that a mutation occurs. This gets exaggerated heavily as you keep cloning down the line. It can lead to a decrease of potency or plant vigor, or it can lead to an increase. Mutations are unpredictable and can occur at any time without warning.

This happens in strains that haven't been stabilized.