1) Medium of exchange.
Bitcoin is only a medium of exchange when merchants accept it and hold onto it. Right now merchants are dumping bitcoin for dollars, so its not being used as a medium of exchange. This btw. also throws the transaction cost benefit argument right out of the window (spread).
Now will merchants ever accept bitcoin and hold onto them?
No.
Why?
Its a matter of perspective, some see it as a medium of exchange, others as a pseudo-commodity that can be held / traded for real currency, in the latter case it is seen as a form of investment and in this way its capability to act as a medium of exchange with merchants bears little relevance. By analogy, merchants do not use gold or other commodities as a medium of exchange, yet they have value.
a) Governments have already effectively made it impossible for people to use bitcoin as a medium of exchange, as they expect taxes to be paid on bitcoin gains. Thus it cant function as a medium of exchange (same goes for gold btw.). Unless Government loses its control on its money monopoly this law wont be altered.
b) Governments will prohibt merchants from accepting bitcoins. Again: They will never give up their monoply on money.
2) Store of Value
After people realize that merchants are not adopting bitcoin and that it is not used as a medium of exchange, it will collapse in price. Will anybody store significant wealth in Bitcoin after this spectacular collapse? Not likely.
Pretty sure someone has corrected you on this point already, there is no single central world bank that regulates currency, as such all major governments would have to be on board to destroy the use of cryptos, in any case your proposition is baseless, there is no way for you to know governments are going to ban bitcoins.
Also Bitcoin is a technology, that can be outcompeted by other altcoins. Your wallet can be hacked. The average citizen cant judge if the code can or can not be hacked and thus more bitcoins created. If you want to store wealth over a long period of time investing in a technology is not the best and easiest option. Also Bitcoin is not useful or wanted. If you ask a Bitcoiner what can I do with it they will say: dump it on the next guy it will go up in price! Gold and Silver where always wanted (jewlery and nowadays industry) and thats why they became MONEY in the first place! They have unique properties and everybody love to hold them in their hand. This will always be the case ensuring significant value and acceptance. Nothing like this can be said for bitcoins. Thus I dont believe bitcoin will function as a store of value in any significant way.
Cool, so basically you're scared, in that case sell your bitcoins and move on ?
NB: It is and has always been the people who provided the impetus for the development and evolution of bitcoins and cryptocurrencies, these people have been staunch supporters of anarchy and anonymity, they have never relied on any government for assistance. Therefore so long as the people will it, use of crypto currencies will rise, but if the people are fearful of governments or a centralized power in general, that fear will allow those in power to walk over those who support such a movement, and destroy cryptos. If you do believe in the movement, and do believe in cryptos you should remain positive and do whatever you can to promote them, if you are only in it to make money, well, I hope you lose it all.