If one day most of countries in the world ban BTC, will someone with a lot of money spend million or even billion of dollars to establish a whole country. In this country, you will exchange with BTC and other crypto. Will you leave your coutry and move to this 'wonderland'?
First you need to find a unclaimed land to make a country, And well there isn't a single piece of unclaimed land left on earth. So I don't think a new country can be established now.
But just to answer your Hypothetical question, Yes I would probably move to that country given that it has everything else that I have in my country, it can't be just a country using btc full of bitcoiners with nothing else like a country.
It isn't that simple. There is technically unclaimed land, but if you claim it, others might as well. Nobody really follows any rules when it comes to that.
There is land on the borders of Serbia and Croatia that neither of the countries claim (they have a claims of other areas that overlap, so they are disputing where a border should be). Some guy claimed that land when he found that out, but all the countries (including those two) say that the land will belong to one of the two countries. He called it Liberland, it wants to use Bitcoin. Of course no one is allowed to go there by border guards and it is mostly a swamp.
You could also make new land, like they do with those palm tree islands in Dubai. You can do it in a middle of the ocean, or just tie some boats together and make a moving city state.
The problem of creating a new country is more political then practical. Humans can do a lot of things if they have a reason, but when others object, this is a problem.