The value of the bitcoin will rise compared to the fiat. If the fiat crashes, the bitcoin price will rise even faster.
I don't believe that myself. There would be a small hard core of believers who'd carry on buying. Everyone else would examine their beliefs, panic like fuck and pour into gold or just sit there and be eaten by devaluation.
Bitcoin's a generation or two away from being a genuine safe haven, if that ever happens at all. For most people this is an extreme bet and it would be the first thing they'd abandon if the shit truly hit the fan.
If the dollar collapses the last thing anyone will give a fuck about is bitcoin. It will be food, ammo and fuel - prepare to protect what you have, take from others, or die. Ffs let's end this ridiculous notion that bitcoin value will moon in the face of US economy collapse.