What are you saying? Are you saying that if the government says that fiats are worthless fiats will not be used? Without fiat, bitcoin and the other digital currencies will be no value, they will be just a digital currency for show. There is no way government will use bitcoin or other currency to be a global currency, digital currency will stay as it is. And I think ethereum had it's limit now, I am not sure, but I think I've read that before somewhere.
not a single country is going to give such kind of statement, even the government will declare bitcoin as legal currency but still they are not going to declare their fiat currency as worthless. it is depending on the interest of the people after long long time if the users of bitcoin consider bitcoin as enough to be use for anything and do not give any preference to fiat currency the there are some chance that fiat will become worthless.