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 low that it can crash and of course you are seeing that Bitcoin is getting more popular and used and that is nice for the currency but there are still a lot of people that is using Dollar.
And that is important for them because that is the only payment method that they are using.