[Fyron]

A big reason that the Roman Empire decayed from within is that they did not have the means to effectively govern such a massive empire from a central location; various provinces (especially far off ones) had too much autonomy, and the emperor/senate had little knowledge of the happenings in such far off provinces. The US has the advantage of instant communications.

Another problem with Rome was that the Senate was not a representative body; it was simply composed of members of the major houses of Rome. This is not the case with the US.

Additionally, the US has frequent changes in leadership with forced elections. This allows for changes in policy every few years. This will at the very least delay any stagnation or fall.