Fiat is already worthless

in what country are you living?
Money is worthless when you can not buy things, food, clothes, ... with it anymore and when you can only use it to burn it to ashes in order to warm your body.
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
in the sense that you're forced to buy with it, but in practice fiat has no value, it's issued by a centrale autorithy that force you to buy things with it, because they made it legal and there are no other option, until now with bitcoin