Using physical metals as the base unit of account and settlement (Exter's Pyramid) allows you to remove all middlemen and if you would like some other type of convenience, you can then build a centralized system on top of it.
So how would build a centralized or decentralized system on top of metal without an enormous amount of counter-party risk? How would you guarantee who held what, for whom, and how much? Goldmoney.com is currently already doing it, but you have to fully trust that they secure your metal and will deliver it in full on demand. And that it will never get seized. HUGE counter-party risk.
Gold backed, nation state issued currency.
Because certainly there was no usury going on when the USD was backed by gold. /s