This is not a very complicated question. FIATS, gold, other form of currencies, and every coin that make up the cryptocurrencies including the BTC, is nothing more than the result of a global consensus in terms of what and where each of these things got the value they have from. The only difference would be when were each of these "items" introduced, because the older it got that global consensus from the time of its onset, it means it is already implanted in the thoughts of many generations, just like gold. For the BTC which is new, compared to the rest, it is also safe to say that part of its value comes from a global consensus, because if the world, as a whole deem it as something that has no value, then by that moment no one would be interested in it whatsoever. Who would buy something that no one is interested with because it has no value? So the answer to your question would be, the same thing that gave value to the btc which is global consensus. Imagine if the entire world decided to ban btc, with not a single country, nation, or government in the planet that acts as a sanctuary for btc, meaning, every nation comes to a global consensus that the btc is banned. By that time, and if it happens, the bitcoin would be illegal, and die out eventually. In the reality we are living in at the moment, the btc is both illegal and legal in a global context, and if we put specific nations under a microscope, you would be able to see that the global context comes from specific nations who either has the btc banned or not. It is as simple as that. The factors leading to a global consensus to ban the btc are the things you should ask. Like, a sudden collapse of the entire crypto realm, related to a cyber attack, wherein each coin holder losses all their coins. Something like that.