Just a minor quibble. The power of attorney is not necessarily rendered ineffective because of incompetence. Pretty much every state now recognizes a durable power of attorney which remains valid even after the principal (the person who granted the power of attorney) becomes incompetent. So if it was a durable power of attorney, it might still be valid. But the guardianship supercedes the power of attorney if there were ever a conflict between an agent holding a power of attorney and a guardian. And indeed the guardian generally may revoke the durable POA that the ward (the incompetent person) granted to the agent. The guardianship has the further advantage that the ward herself cannot overrule the guardian.