Above the fact that no sane government will allow a decentralized, non regulatable currency to be 'the' currency', bitcoin and fiat don't really serve the same purpose. Yeah, they are both money, duh, but you can't hold a bitcoin in your hand. Nor can you make a bitcoin transaction while you don't have an internet connection. As useful as bitcoin is, its benefits rapidly degrade in a mostly offline world. Physical possession of the token (fiat) will always trump some numbers on a blockchain.
The wallet in your jeans pocket will never get stolen because of a virus. The 20 bucks fiat you have will still be 20 bucks when you wake up the next day. And most importantly to me, there is not a giant ass online record of what's in my wallet that everyone can peruse at their leisure