The only thing that will make USD crash is if there is a mighty, global economic collapse. That is not good news, even for bitcoins.
It means bitcoins will be the only reasonable means of exchange and you can't even sell btc for your local currency because your local currency would have been seriously devalued due to the world economic condition.
If there is a global crash, the dollar will rise - flight to safety. It happened during the financial crisis of 2008, and then in 2013 during the eurozone crisis - the dollar rose both times.
The dollar isn't going anywhere (and neither is the pound or swiss franc). They are backed by stable democracies, and yes they will have recessions from time to time because recessions are normal, but nothing is going to happen to their currencies.
It is the weak countries that will adopt bitcoin. What will happen is their own currencies become worthless but they can't get hold of US dollars (because of capital controls), so they will opt for bitcoin as the next best thing. See Venezuela for this in action. The bolivar has collapsed, the people want US dollars, they can't get them, so some are opting for btc.