Dollar has exchange rate between it and other currencies. Also dollar's value is changing, and economists use various methods to calculate this change. There is not a single thing in this world that has a fixed value, because value in market economy is subjective and unstable. Just because people like to measure everything in dollars, doesn't mean that dollar is vastly different from other currencies. So if Bitcoin replaced the US dollar, it would still have a price in US dollar and other fiat currencies.