Either humanity change a lot, of most organizations will underpay the people doing network maintenance and the secrets will be poorly protected even in important organizations. Humans are poor at things like creating and remembering passwords and untrained humans are poor /uncaring at detecting stuff like phising attacks.
That computer security in a large organization is going to be poor will mostly what you have to expect, and that a network is solid and well defended and not penetrated more a exception, so acting as if all networks are already been penetrated could be reasonable. Then what you do? is all to you. Is couriers a solution? I dunno.
removed useless text to get to the point better