You're simply implying that when Central Bank collapses and together with the Universal Fiat, US Dollars, cryptocurrencies will have a chance to take over. Well, for me, I dont think so. Why? what about bigger countries other than US? What about British pound, CNY, Dinar or Japan currency? Are they not considered to be influential enough and has the capacity to take over? Think again.

Ill give you a better example. Assuming all countries boycotted the US and its financial system, China could take over, because China is one of the most biggest supplier of raw materials and that will boost the value of their currency, Yuan. Well the goodnews is they adopt bitcoin and other cryptocurrency now. If youre a chineae business man and you choose to use cryptocurrency, can you buy goods to a certain country that never appreciates the value of a crypto? Well, you can't. And that will force you to use the FIAT. See the Picture? You just can't destroy a certain currency that we all been using the whole time, not now, not later.

For my conclusion, I could say that cryptocurrencies are Goods to buy Goods; in a way that it will be more faster, convenient, with zero fees and lastly decentralized.
Thanks for reading guys. :)