They can still ban it completely through centralized exchanges. By cutting off people's access to BTC with Fiat, it will be much harder for the cryptocurrency to be used at a large scale worldwide. The only solution would be to acquire BTC in person or through a decentralized exchange. But if Bitcoin is declared illegal for mainstream use, then people will think twice before using other methods to buy it with Fiat. They could easily face fines, jail time, or execution (depending on the country).

At least we know that not all countries of the world will turn themselves against Bitcoin. Some will support it, while others will reject it. As long as there's some level of support for BTC, we can be sure it won't go away anytime soon. Who knows what the future holds for the cryptocurrency? Just my thoughts