Can bitcoins replace conventional money? No, for the simple reason that governments will never accept bitcoins for paying taxes. As long as you have to pay taxes, you will need your national currency for that (except if you can convince the government of your country to give up the national currency, or to abolish taxes altogether, which seems unlikely).

Can bitcoins complement conventional money? Yes, they have already started doing so and will hopefully do so to a larger extent in the future.

Also, pluralistic systems have always proven more successful than monolithic ones. Why not have pluralism with respect to currencies?