Suppose a country began to use Bitcoin instead of their currency. Then, what will happen?
A country can never totally abandon to use their own currency and start using Bitcoin alone. In countries where Bitcoin is legally allowed, there are some individuals that are using Bitcoin for their daily transactions. The government of El Salvador has made Bitcoin ba legal tender but that doesn't mean they will stop using their own currency. So, I doubt if there's any country that will totally forsake to use their own currency even if Bitcoin is a legal tender in that country.
There are several reasons why countries will never abandon their own local currency perhaps I don't think if that was even part of Satoshi's reasons for inventing Bitcoin, if clarity can be made and to play fair justification the question is if Bitcoin which is digital is made to abolish people's locally currency what then will the faith of those that are not literate or in those areas where there is no Internet facilities in accessing Bitcoin as a country currency? So you see some time people should should feel that every one is sharing the same privileges they are in light of making of certain assumptions or consideration, Bitcoin and every other locally currency will keep working hand in hand for their purposes and never to be seen as a fight.
You’re right that the sole purpose of Bitcoin isn’t to eradicate or replace the traditional fiat currency but to offer some sort of a safety net that people can fall back on incase they wish to protect their money from inflation, since Bitcoin is immune to inflation. There are several purposes that Bitcoin serves and several other purposes the existence of Bitcoin plans to achieve, and completely replacing the fiat system isn’t one of those purpose. Yes, Bitcoin is still relatively a new technology and many people are yet to hear and get involved with it, so if Bitcoin eventually replaces the traditional fiat system, so many people would suffer, and even when they manage to get involved, they’ll be an easy target for scammers since they’re are still very naive and doesn’t know much about the technology.