I've been gaming it out and I see no way for it to coexist as it is a direct adversary to the global central banking system we exist in. Crypto eventually causes a global run on the banks once we reach a tipping point where average people start demanding crypto over fiat. This triggers a derivatives explosion (1.3 quadrillion dollar market) the global financial system implodes and all fiat debts are erased. What is left is complete chaos all public services go unfunded, pensions evaporate, companies go under due to their fiat assets evaporating. A new order arises but who the fuck knows that that will look like, world war 3 style military actions begin.
I do see government intervention before it goes that far and it will be the draconian kind. They cannot coexist since crypto cannot be centrally controlled , regulated or tracked due to cross chain interactions which are undetectable. I only see one path for them and that is waging war on crypto public chains. Long jail time for hosting or participating, extradition and global cooperation since not cooperating might as well be a declaration of war.
I doubt that cryptocurrencies would replace the fiat system that we have though it can stand together in the economic growth of a country but if so then everything would take slowly with systatic and orderly transition from fiat to cryptocurrency.
I doubt your logic really because people and the government would prefer a stable medium of daily transactions in their respective countries and Bitcoin's future will be used in a global market and more in online world which people prefer to invest to it and use it as a online money or currency.
The point is fiat and Bitcoin will work together having its uses rooting from the real and virtual world which will benefit both the country and its people.