What do you think is the likelihood of a 2008 financial crisis happening again if we only used bitcoin and there were no fiat money at all?
you have forgotten other factors that could affect the global economy. There are assets that have value, gold, silver, precious metals and other assets that can replace paper money.
Also, if all the support for Bitcoin moves, this will affect the cryptocurrency market and the effect will not be %%% on the rest of the currencies.