Considering that there are 21M bitcoins to be ever exist(I didn't count all of the lost bitcoins that may be out there which is a number that is higher then 2-3 M for sure), then the total value of all the bitcoins would be 210B$, which is less then 0.01% of the total fiat that exists.
If you want to make fiat worthless, we will need the bitcoin price to be something around 50M$ which will also make 1mbtc to be worth 50K $ which is simply crazy.
Exactly, it is something really crazy at the first place, because the very first thing to be considered is that bitcoins are limited, there are only 21M bitcoins including the lost ones, so i don't feel even if the price of one bitcoin reaches to the price mentioned it would make fiat worthless, because only the price is not everything but the economy should be seen.