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.