If BTC was the world's leading currency...

Government can't put tax (or charge, I'm not sure about the verb) on it.
Black market will have an intractable currency.
We can have transactions with no one know about it, so we will me more free. But do we really want to be free ?

I don't think BTC can be the world's leading currency. But it could be the black market's leading currency. And everybody can buy and sell on black market.