Sounds like a great little analogy, until you realise that comparing the transfer value to asset value is meaningless
Instead, you need to be comparing the value that is securing the value being held - this is economic security
You don’t mail a trillion dollars because there is not sufficient security
And of course it doesn’t work for a post office because a post office is not a digital asset requiring security
And all of this is by the by, because ETH is the mail and the post office at the same time