A sudden and severe blockage in global trade due to the lack of sound money in a "dollar collapse" scenario would damage the supply chains that provide the constant flow of energy and goods into the world's major cities.  Without sufficient energy and goods, society in heavily-populated areas would break down.  

In a dollar-collapse scenario, gold is too slow to facilitate global trade if it has to move and requires too much trust to facilitate global trade if it doesn't.

To permit a quick transition away from debt-burdened fiat currency without a breakdown of order in the cities, there would need to be a robust proven alternative in place at the time of collapse.  Political institutions like the IMF will not be trusted, as power structures will be fragmenting.  One would need some sort of trustless money, sort of like gold, but with the ability to quickly send it across the world in order to keep the vital trade lines moving.  It would be helpful if this money already had a sufficiently broad user base so the most communities had members familiar with how it worked, and a sufficiently established billion-dollar network of globally-connected hardware that would stand up to turmoil during the transition.  

With something like that, the transition away from fiat currencies could seem anticlimactic in hindsight.