Agree that the risk is low, but a derelict account still increases the attack surface with no benefits. Who knows what undiscovered vulnerabilities or ways to exploit a dead account will pop up tomorrow? Security is always a balance between cost, convenience and safety. Deleting a dead account yields a bit of security at almost no effort, and no loss of convenience. Sounds like a good deal to me.
Best, Marv
Marvin Waschke