Only cryptocurrency or single bitcoin will never be the single currency system of any country. If bitcoin is permitted to use, present system will also exist. We know, many countries have already permitted bitcoin to use and gradually this currency is enlarging its own place.
Both are existing. Some countries may ban its use but soon enough with pressure coming from other countries and their own people they will also allow crypto usage. The point is not that, but that bitcoin's dominance of world's economy is very small. The biggest portion is fiat.
With the development of cryptocurrency, blockchain technology is also developing day by day.
Blockchain is the technology behind bitcoin. It is not synonymous with bitcoin.
So, I think, when most of the countries will permit bitcoin, a controlling system of the countries will also be introduced in this blockchain technology.
The essence of bitcoin comes from its property of being decentralized and censorship-resistant. I dont know how a controlling body can be made.