" Contrary to the commonly held belief that this is fundamentally impossible, we propose several solutions that do achieve a reasonable level of double spending prevention"
Yes, that's Bitcoin's claim to fame.
Perhaps the number of zeroes doesn't make a difference; but solving the double spending problem would seem to play a much bigger role in making a difference to your conclusion from ten years ago. Note that one of the concepts around the double spending problem is that of offline spending being able to happen in massively large scale in very short time before the network is rejoined; however, in the case of email, that situation is largely a dead end; if you're not online, you're not going to be making very many mail connections.
If you actually care about this, you might consider what would happen to the Bitcoin blockchain if it were attacked with millions of double spending transactions. This paper claims it can't prevent double spending, only prevent overspending by a factor of 100, which may be of theoretical interest but isn't of much practical use. We already know how to do approximate bulk counting. Oh, and on the last page, they think that hashcash works, to limit transaction rates. Anyway, if you reread my paper from a decade ago, the bank problem is only one of many problems with e-postage, each of which is fatal. R's, John