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.
Correct, 10k for bitcoin is not enough for the world economy to function because we know there is a limit in the number of bitcoin, I have not done the math but bitcoin will probably need to reach at least 1 million per bitcoin to be able to run the economies of the world.
Good observation. Also, may I add that Cash has unique properties because it is a physical token of value. Many transactions made are not made to be seen by the public eye, and cash facilitates this, it is the ultimate anonymous value transfer mechanism. Also, if it became a world currency, we would see individual governments have unique currencies. They would not late adopt into a global currency.
The government don't care about the 1 dollar, two dollar transaction of yours. They can't squeeze anything from that. But if it's something in millions or billions, even if you do it in a private transaction with the physical form of fiat, you might have a hard time. For example, if you deposit a million dollars into your bank account all of a sudden without you getting promoted or there's basically no change in your source of income, you'd see an agent knocking on your door the next morning.
If you choose to avoid that and you so you chose to just hide the money in your own vault at home, then you will be doing money laundering (if it was obtained illegally) or money hoarding (not sure if it's illegal in your place) which is, again, against the law.