Bitcoin is digital currency and also one of the commodity like gold. It cannot be replaced with USD, still major countries are not still not accepting bitcoin as one of their option. It will take lot of time to establish a digital currency into physical one.
Yes there no need to be compared because USd will remain as USD and being used and popular whole over the world. Unlike bitcoin, it is still on its young age and needs to be adopted by a lot of people. But for us, that who are already knew it both of them are very important to us because we are able to convert our bitcoins with USD.
Yeah it may take bitcoin quite some time to have itself fully established as it needs support from countries' governments and banks. Although bitcoin is slowly but surely being known in a lot of areas, the level of use is still dwarfed by that of other fiat currencies