When the dollar collapses, if it collapses, then chances are the best interim medium of exchange is going to be gold. It is a resource that has recognized value everywhere, and it is most definitely something people are willing to pay for. Bitcoin, on the other hand, is more niche and not as exchangeable as a PM such as gold. While Bitcoin is great for a society like the one we have, it doesn't really prosper without a highly connected community.
In order to be considered for a medium of exchange, the replacement would have to have a fairly stable purchasing power.
Gold will not have a stable purchasing power in the USD collapse scenario and neither will Bitcoin. The US gov't will enact measures in an attempt to stabilize the currency and it will continue to be the medium of exchange. Failing that, perhaps the Yuan, the Ruble, or (heaven forbid) the Euro will replace it but the alternative will most definitely be a fiat currency. In this scenario, gold will be hoarded and its value will moon (wrt any fiat currency).
Bitcoin is, of course, off the table
But with gold it remains to be seen. If the US dollar crashes, things go from bad to worst with the US itself being dissolved, and there is a need for a currency intended to settle international accounts (which is a big if, as I said before), the governments may agree to return to a kind of gold exchange standard (new Bretton Woods but now without the US) and set up a unit of account backed up by physical gold, which would be used on an international scale for clearing trading imbalances between countries
Just kidding.
