bitcoin will never be able to suck all the fiat money, they can just print more without problem
also fiat is already worthless, but you're obligated to use it for the majority of things
if you mean collapsed, i don't think so
how can it be worthless from now onwards .the whole global is running on fiat and some places accept bitcoins and other digital currency .and it is long way to go when fiat will become worthless.