Without central banks, bitcoins won't exist. They are both in a different class of their own. None have to replace the other. Why should they? Irrespective of what we thing of the government and global financial institutions. I don't mean to sound pessimistic but if bitcoin "kills" central banks the world would experience chaos like never before besides I believe this was never the intention by Satoshi when he/they created bitcoin. We really have to chill and stop sounding like the group on Reddit Forum that want to see bitcoin die.
Both can complement each other, many things we can do with bitcoin but fiat currency can't do and vice versa, I think what bitcoin has achieved today is a positive thing that can make central bank awareness to adopt the positive side of bitcoin, for example, transactions that are transparent, low cost and of course faster.