How approaching Black Swans could deter a convoy attack

A submarine commander could decide that attacking a reinforced convoy was too dangerous and withdraw before weapons were fired. That possibility gives an expert escort group a protective effect that a list of sinkings would miss. The Black Swans' approach at speed is used to illustrate how an attacker might become aware that reinforcements were arriving. This is an interpretation of deterrence, without a count of attacks demonstrably prevented. It also complements the separate morale effect on friendly escorts that expected help.