The advantage fiat has over crypto is usability, most elderly persons and illiterates will have difficulty in interacting with crypto and securing their wallets.
A very key point you just mentioned, which the opposite term should probably be acceptability. Definitely, everyone can use the US dollar, but it would now depend on who chooses to accept it. The fiat of a particular country is acceptable in every part of that country but might not be accepted in other countries except with the intervention of the bank were exchange is been made.
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.
Both of them are quite unique in their own way. Bitcoin gains value by accumulating in price but fiat doesn't.