It is critical infrastructure. There are two parts: a.) mimic the thing our predecessors created: postal mail was so important that they had a person on a steed ride across barren land for days to deliver a message; b.) how do we adapte the modern equivalent. It's not so hideous for someone to make a copy of the payload on the horse, is it? It's not good to copy it. But it's much worse if the original is compromised: faked message or faked originator or destroyed.

It demonstrably doesn't fulfil that need; GP's point, I think, is that postal mail has (perhaps more obvious, especially outside HN) vulnerabilities that were coped with for millennia.