They lost me here. Most countries indeed have currency that is backed by their nation's respective treasuries. However, those treasuries are filled dollars, euros and yen. All fiat money no longer backed by gold or anything tangible and whose values are based solely convincing others they have value.
You are arguing that trusting issuers has been societal valuable, which sure - historically it has been for the fiat currencies that still exist (if you ignore survivorship bias).
Not sure what handwaving "society is valuable and that value is what backs fiat currencies" even means in concrete terms. The government has the power to print as much fiat currency as they want (and they frequently do with intentional inflation targets). There is nothing that stops governments from printing more money except fear that it will damage trust in their currency, leading to devaluation.
You can continue to assert this, but that doesn’t make it true. And it is definitely not “hand waving” to make the observation that organizational structures provide value. Some more than others, depending on the context and time. Fiat currency arises out of the dominant current organization into nation-states, and the value is from the same root. It’s more or less an emergent property, not the directed efforts of some few elites to convince everyone else of the value. If and when this organization loses value, we’d likely see a similar devaluation of fiat currency. For example, a “mad max” apocalypse scenario.
The reason people believe in the worth of cryptocurrencies is greed and fear of missing out.