The problem when the national currencies arise would be more about regulations than anything else. Just to give an example banks have a bigger power over the monetary system than they should, why bitcoin was created was to stop this overpowering and making sure that everyone is equal when it comes to money and finances.
Now, if the nations build their own stablecoins that would be great and be better than all the ones we have but if they do not regulate it properly they could literally use all the money they printed out of thin air to buy crypto and basically corner the market themselves. Think of USA and China having a centralized exchange? That would basically mean end of the world because they could collect like millions of bitcoin and even more altcoins all on their storage and they could simply delete them all and ruin crypto world.
The good thing I am thinking about this is that cryptocurrency will be known all over the country and that makes the cryptocurrency being adopted by a particular country. And the other side or bad thing about this is to the one who is using it, although it can make their transaction faster and safer that way, the government can collect some taxes maybe if that happens. It can make cryptocurrency in a particular country be centralized because they can regulate it whenever or whatever they want.
I think that can be possible to happen if every country will accept cryptocurrency. Besides that, every government can learn about crypto because without having enough knowledge about cryptocurrency, they cannot create their own coin. With the developing of technology, we might see a big chance in the transaction furthermore about digital currencies which are already happening in many countries. The government will make new regulation about cryptocurrency because their coins will be used as another option besides of other coins.
Indeed, it is not possible as of now because as of today cryptocurrency is still banned from some other countries with their own reasons to regulate it. But if they actually legalize cryptocurrency, the community itself will rise and so do to the price of the market itself maybe.