The only advantage of fiat is it is widely accepted but Bitcoin and other cryptos are also going to that point.
As fiat has been made as legal tender and backed by the government gived that advantage.
As for BTC, we as community backs it but an advantage of btc as highly volatile cant be compared to fiat.
Everyone is not familiar with bitcoin as Bitcoin is not a famous currency in their consideration for all humans all over the world because some of them don't know about Bitcoin and if some know about investment then they don't believe it because it is volatile and unstable currency where as fiat currency is widely accepted as it is used in all aspects of life.
People will accept Bitcoin but they also cannot leave fiat currency because that is used for purchasing daily objects. I think that both have its own importance and we cannot ignore the importance of any of these but we have to take advantage of both according to its uses.