I have not said anywhere there is only 1 Fiat currency?. If the U.S economy collapses, do you honestly think the world economy as a whole would be fine, and that Bitcoin would remain at '$2500'?

Bitcoin is not Gold, and never will be. Gold can be traded physically on the streets. If the 'world ends', people will be trading Gold, Silver and Copper as I reckon the lack of availability of Starbucks WiFi may hinder the long and expensive Bitcoin transactions.

Bitcoin's price is generated by the total mean price against all currency trades. When It has it's own unique price, in which my house will say be worth 150 BTC and not £300k , then I will agree with you more. Until then it remains inflated and controlled by the world economies.