There is no guarantee that any item will be worth any amount of another item. You can make claims about the relative likelihood that some item will be exchangeable for some quantity of another item, but you cannot claim that in every future scenario your item will be exchangeable for any amount of another item.
Just because a transaction looks more like barter than commercial doesn't magically convert the ideas underpinning fundamental economics. At this level of understanding economics is more a study of sociological systems and less about banking, fiat currencies, and all the other modern mumbo-jumbo we go on about these days.
This is really key when discussing some utopian reality where we've untethered ourselves from any form of currency--an exchange of value is tied to the very earliest societal systems. Unless society shrinks back into nomadic wandering tribes some form of value exchange (aka: currency) will exist.
I've seen a lot of good arguments on the value/worthlessness of fiat currency, but I'd never seen fiat currency defended as having the value of not going to jail. That's an amusing and on its face compelling argument.
Actually this rule isn't limited to taxes. EVERY debt you may have in the united states HAS to accept a pre-agreed dollar amount as payment in full.
The value of the dollar is "guaranteed", in theory, by the law that states that anything for sale inside the borders of the united states can be bought for dollars, without exceptions.
As to how guaranteed that makes a dollar, that is a matter of opinion. It is a lot better than Bitcoin imho.
