Bitcoin is born global and traded against many different currencies. If one or a few crashed then it will affect the relative price but doesn’t change the value of the underlying system. If the crashing currency was a significant trading pair i.e. USD then you will definitely see fluctuation but there would remain a finite availability of an asset which could arguably be even more in demand. If it dropped too far people using other currencies would see it as cheap and if it raised too fast then people trading in other currencies would likely divest into other assets. Theoretically the market should self-correct without manipulation and whilst locally there would be a period of price discovery which could be a wild ride, there is no reason products and services couldn’t be priced in Bitcoin. Over time people would come to accept the new values as we have with the dollar from the trading classes that came before it. There are multiple examples of currencies collapsing and foreign currency taking the primary role within a country. You may not pay the exact same price for a product in two locations due to supply and demand amongst other factor, but the base unit of the currency is still the same the base unit no matter where it is used.