Some countries were already banned bitcoin, and here in my country they did not banned it not because they don't know what to do but they find bitcoin as an interesting and a great opportunity for the people, and a big help in our growing economy.
I agree, it's not actually a "what if" anymore, there are many countries that have already banned bitcoins and other related activities, although some, not in its entirety, only in different aspects. However, if most or many countries would ban cryptocurrencies, then it would really hit the market big time because that's one big chunk of where the support the market needs is coming from. The support the community gives it is really a game changer, and it can make or break not just coins, on their own, but the market in its totality. However, in contrast, if more governments would see its beauty and help it grow, for sure, it would bring out something good.