Given the fact that, for example, the price of Bitcoin over the past year fell from $ 20,000 to $ 2,500, what could happen to a country that begins to fully use Bitcoin and replace its national currency with cryptocurrency?
The price hasn't gone below $3000 on any of the more reputable exchanges. You either made a typo or looked at an illiquid exchange chart.
I don't believe that Bitcoin will replace national currencies. It might become a supplement to Gold, but then in digital form. Governments holding large bags of something incentivizes them to support that something so that their bags don't suffer too much. They have an excellent form of money they print out of thin air that they can use for that specific purpose.
The ECB this year in a
tweet said;
Praet: As a central bank, we can create money to buy assets
Governments could issue their USDT like token on Bitcoin and use that as money, but you still have to deal with a very slow and non scalable underlying network.
It will serve better as alternative to fiat currency and then could also serve as reserve too but would never replace the fiat or completely replace the present currency for now, but in future when technology becomes wide and we have no illiterates any longer in any part of the world, then fiat could then be replaced with cryptocurrency, and when fiat is replaced, its worth will no longer be measured in fiat but in the quantity of the currency itself, and maybe all these gold are what will start getting being measured in fiat, and the bitcoin itself will later serve as store of value in future because the number of people holding this coin is really going and the more they accumulate these coin, the more it reduces its supply in the market which makes it expensive and stable.