Oh, they will find ways to pressure the other countries to get in line with the plan, I'm sure. Except a few may be. Like North Korea, but who gives a fart what they do. Even the Chinese will have to carefully decide - after all, they live from the trade with the west.

Making BTC illegal doesn't neccessarily mean to declare it void from one day to the next. Probably they will give investors a chance to change it into the new legal crypto, if the origin of the money is cleared and all that.
But once its illegal, nobody will touch it any longer. Why would anybody want it? You can't buy anything with it, or change it into a legal currency, and you don't get high from it either. Money that doesn't buy anything is just meaningless numbers on a piece of paper.