If I need to "trust" you that I can get <something> in exchange for my cryptocurrency, then that's not much of a cryptocurrency is it? It may as well be a MySQL database controlled by you, as that's what it amounts to anyway.
The problem is that that's not really a currency. It's zillions of IOUs. I don't know how to make transactions between two nodes that don't trust each other or how to come up with one number for someone's net worth.
> Graeber proposes that money as a unit of account was invented the moment when the unquantifiable obligation "I owe you one" transformed into the quantifiable notion of "I owe you one unit of something". In this view, money emerged first as credit and only later acquired the functions of a medium of exchange and a store of value. [1]
Like chips in a casino can be exchanged with the house for cash, but I don't need to tell the house that I'm giving some chips to my friend.