[adyus]

The (U.S.) dollar is backed by the U.S. Army and always will be. Confidence decreases? The U.S. will likely start a war with the country whose currency tries to replace it (on "national security" reasons).

Fiat will always be backed by military force.

[smeej]

Well there's your problem: Assuming it will be a country's currency.

[adyus]

In that scenario, trust shifts to somewhere the U.S. cannot easily regulate. In case of crypto, that can just be banned and rendered useless, as long as the rest of the world is USD-bound.

If a crypocurrency becomes popular in another country using another currency to on- and off-board, see my comment.

It would take a miracle for cryptocurrencies to suddenly replace fiat without being bound to any fiat currency. Essentially, all the world would need to switch trust at once.