I do not understand why I should compare these currencies. The dollar is for the government, and bitcoin is for the people. So it was from the very beginning of the existence of bitcoin. And bitcoin will never replace the dollar
It can't be compared in all of the conditions. eaither are different commodity. USD as a legal fiat money can't be threatened by bitcoin as illegal commodity based on the goverment's regulation.
Both are need each other. There is no reason to compare both of the currency. remember bitcoin price is pegged with the dollar itself.