I do not think so highly Bitcoin soar to 10,000 although I hope so) While earlier people did not think that he will be worth $ 10
Anything is possible, it may reach some day there in future, but even if we see that high price I don't think fiat will be worthless as fiat is basic necessity of many people.
yeah I agree with fiat is the main currency for all human although other foreign currencies has a high price then it will not be too influential for fiat. unless the bitcoin users continues to grow and all the people on earth using bitcoin then it will give effect to the fiat. but something that would be very difficult to happen