The possibility of cryptocurrencies being banned in the U.S. is probably greater than what many people realize, especially those current holders. At least an attempted ban not unlike what happened in India.

While the whole "illicit transaction" argument is bullshit, Yellen and other Treasury officials probably realize that cryptocurrencies like Bitcoin could pose an existential threat to the U.S. dollar and needs to be snuffed out early. Companies like Microstrategy and Tesla have already began using them as substitutes for U.S. treasuries. Together with the decline of oil dependency and the petrodollar, a day will come when the de facto currency is no longer the U.S. dollar and with it goes the world currency status. Yellen's department, the U.S. Treasury, would become a minor department and a shadow of its current self.

An apt analogy could be the U.S. government is like Nokia in the early days of the iPhone.

That said, a U.S. ban may actually unintentionally lead to greater adoption as it effectively means cryptocurrencies withstood and prevailed an attack by the world's foremost national power.