Patently false, and showing your ignorance. In fact many countries have or have had competing currencies along with functioning economies. Europe (yes not a country, but proximity makes it the same) for a long time had competing currencies, there are Latin American countries now that allow for competing currencies.
Your argument assumes haggling (assuming that to be bad), as opposed to people being familiar with the currencies. You are using almost every possible fallacy contrived to make your arguments, and they've all been dealt with. Should we all be forced to deal with monopolies in other markets? In the meantime we are taxed for using the current fiat money and you argue this is good?