Who knows? First, it is difficult to make any sort of comparison to the Roman Empire for a thousand reasons. Niall is right in stating, that it basically launched Europe into the Dark Ages with little impact on the rest of the planet. The British Empire has fallen with little impact on the state of global society, as there were others there to pick up the ball. So even, a substantive decline or fall of the American Empire is not of great concern to global society, as there are many in the wings ready to step in and provide a leading role on a combined or singular basis. The world will go on.
The bigger concern is the fall of the "global industrial-financial complex" that is not run by geographic political states, but a collection of multi-national entities operating without borders. It is an invisible political state that operates organically without the protocols that have defined geographic political states. It is the collective inertia, infrastructures and outputs of the business and economic community for our global village. It is a ghostly enigma, but nonetheless exists by virtue of long-standing practises and inter-connections that are entrenched among nations around the planet.
It has no flag, no symbols, no constitution, no formal codes and no figurehead as leader. It cannot be attacked geographically or otherwise.Yet, as world leaders certainly come to understand, it is an entity that must be recognized and dealt with using practical diplomacy. It is topic reaching far beyond these comments.
The point being, it is when this "Invisible Empire" falls, then humanity as a whole would enters a Dark Age similar to regional historic periods. Triggers for this collapse, could come from breakdowns in global finance, international trade, bio-sphere devastation or the shortages of the physical ingredients required to keep a "global industrial-financial complex" running. Most likely, any of these cases or combinations, would lead to horrific global hostilities.
The next dark age may occur sooner than any of us expect, lasting longer than can be imagined. This planet is a small island in the universe, isolated from neighboring entities offering alternative possibilities for existence. And we have seen what happens to small isolated islands.
Through-out history empires rise and fall, there is little to believe that this one is any different - a sobering thought.
The Invisible Empire?