Bitcoin could reach 10k without fiat having to go through hyper inflation or anything, if adoption grew by 10 times the current amount of people and business, then it stands to reason to believe that the price will grow that much, so fiat will not be worthless in that scenario.
Well the price suggested here is ridiculously low for the fiat price to become worthless. Multiplying the adoption of Bitcoin by 10 would still leave it an absolutely tiny currency compared to, for example, the US dollar.