The mass is giving it value, because they know that their fiat currencies are usable as money everywhere, and not just in some lucky places like crypto currencies, meaning, we have a very long journey of acceptance to go through.

Fiat's main perk is that you don't have to bother looking at its value on a daily basis to know what you can buy with it. I know, some fiat currencies suck hard in that regard, but the more stable ones definitely do their job well.

What happens with fiat when we suddenly realize that we can't spend or exchange it anymore? It becomes worthless. It's not backed by anything that would allow it to maintain somewhat of a base value. Fiat value is in our minds.