[vlehto]

Nah.

The real problem was the Marian reforms. With professional army the imperium was able to expand to a size that was impractical to rule. Also success was so thorough that there were no strong enemies left to practice your troops against. And no enemies that would weld the society into single cohesive nation. War protects from despotism, as Kant said. When you don't have to fear war, the wannabe despots start to rob what they can.

Hadrian only saved what he could. And giving the empire some 400 years more life as he did so. And the walls of Constantinople lasted still thousand years after Hadrian had led the way of wall building.

Now if you wish to connect dots to possible downfall of U.S. Then the critical things would be instituting selective service after Vietnam war. And then collapse of soviet union and now U.S. is also lacking credible enemy. With any luck China will quickly rise as competitor.