If a global disaster occur. I think noone can ever use bitcoin. Because its a global disaster and all of us will be affected. So we all we need is hold our bitcoin. And we don't know if its price will increase or decrease.
Well are yet to experience any sort of global disaster so I will not give that verdict yet.But from what I know from history, is that in the face of global disasters, Gold is the only commodity who's value seems to increase.
Definitely global disaster will decrease the bitcoin value because at that time people would have no extra money to put in bitcoin and when bitcoin receives no investment the value will go down and so will be price. In time of disaster people would think only to save their selves and you are talking about the value of bitcoin. May there be no internet and no cryptocurrency.