I can't imagine how the world would be if bitcoin will replace fiat.
What happen to the government then, they will print money which they can control, if bitcoin will replace fiat, then there will be no control, only the big holders can control the market by moving it and I doubt the government would allow that as they always want stability, hence fiat will stay.
Move it against what?
Have you forgotten that in your scenarios there is no more fiat, no more USD no more Euros?
So, against what will the whales move
BTC up and down?
Imagine a world with only one currency, how could you manipulate the currency against something that exists no more, exactly what you're talking about happened to Greece when they couldn't devaluate their currency anymore and weren't able to print money to get out of it.
Bitcoin will never replace FIAT as a currency because it's been badly broken (no longer a cash payment system) and was converted into a settlement layer by the disgusting neckbeards at blockstream.
Says the disgusting neckbeard who was claiming around here that he knew and was Satoshi's friend.