Rome didn't just collapse, it was sacked. The US is incapable of suffering a fall of Rome. America's geography makes that impossible. At the very worse you would see a return to a more insular America like it was before WW1. But even without American hegemony, the nation would still be a major power being the world's largest oil producer, the world's largest agricultural producer, and the world's largest consumer economy.

The problem with any loss of American power is that potential great powers that could replace it, such as China, are deathly reliant on American lead order. China is neither energy nor food secure. How will they manage when their shipping isn't defended by the USN? How will they manage when climate change strains available resources? Especially when they are so geographically isolated from potential sources of those resources, and rivals are closer to those resources.