this assumption is not right because nothing can replace the whole world's currencies since nobody is going to agree with that. besides bitcoin is not capable of doing that.
But, there's a big problem here, bitcoin requires electricity and global internet to function well, and these kinds of services probably won't be so stable without working government.
other things need electricity too, banks, ATMs, printing money,...
if we assume bitcoin replaces anything, it will also replace the power consumption by those things too.
So... bitcoin substitutes currency, government is destabilized, society descends (or ascends?) into anarcy, electricity & internet become unstable, society descends into the next dark age. Has anyone thought about that?
government is not powerful with fiat, yes they are printing money but that doesn't mean anything in my opinion. you are overthinking things into chaos.