You are forced to pay taxes in fiat. USA (and other countries) have a powerful army they use to back fiat. States offers financial titles and other stuff. Fiat is backed by many ways.
So you say that the "use value" of fiat is that USA accept it as payment of taxes, but this is an "exchange value" not a use value.
Fiat has no use value, unlike gold and bitcoin.
Once the population discredit their own government and stop to pay taxes because they become persuaded of the immorality of their government, then your fiat lose this value.
This can't happen with Bitcoin nor Gold.
Which is why I think there is more chance that Euro fall than Bitcoin.
With the Blockchain, as services develops on top of it everyday for other thing than monetary use, you are sure you'll be able to sell your Bitcoin because people will always need to insert in the blockchain.
As with Gold, that can always be used industrially or for jewel.
Such use value is very important because despite of the volatility or manipulation of the market, you know that what you have will be useful anyway, independently of its exchange value.