For me, it's impossible US Dollar would crashes. Even if it crashes, other currency will take over US Dollar place, like Euro or Yuan.
I don't think bitcoin could replace it.
Fiat will just be replace by fiat also,when the dollar crashes most likely the euro will replace it as the top fiat currency in the world.
The chance is really small that the US Dollar can crash and that is because it is still many used so that is nice for the dollar itself and you see of course that Bitcoin is getting more used and that is nice.
But one thing that I can say for sure is that Bitcoin will be the most used payment method in the future.