I think it might happen, but the problem is "does the government in a country want to accept Crypto to replace a country's symbol, which is to replace currency"?. I think that is the problem that Crypto Btc and Ethereum cannot replace a country's Fiat currency.
That's a very difficult thing to replace because it still needs special negotiations by all parties involved in the government in a country, because every country also has laws that have been written neatly and have also been implemented by all of its residents, so when there is a change or mutations in finance, then this will cause confusion in rural communities who are not familiar with Bitcoin and Ethereum.
Yes, it's true, it seems that in terms of replacing a currency in a country, it is not enough to just negotiate in parliament !!, but there must be many parties involved in it and there will be controversy, this is part of several reasons that will not materialize to replace Fiat currency . let alone replace it with Crypto, it won't run smoothly at all because of the Fluctuations that Crypto has.