I think your analogy is flawed. You assume that any attacker that breaches a layer of defense will do so by breaking the encryption or by stealing the key, but

I think your analogy is flawed. You assume that any attacker that breaches a layer of defense will do so by breaking the encryption or by stealing the key, but that doesn't have to be the case. In your analogy of the bank robbers, the robbers might exploit a weakness in an exterior wall, and gain entrance to the lobby. The other locked doors will still prevent them from robbing the vault.