What are you basing this on? The patrician families continued to rein under the Empire. Which side do you think backed Sulla’s dictatorship? Which side do you think felt (and feels) constrained by elections and democratic norms?
> It is Pax Americana, and every empire imploded whenever it stopped expanding
One, source? Because plenty of great civilisations across history reached stable states for hundreds if not thousands of years.
And two, America is still expanding. The economy is growing. Post industrialisation, there are more routes to goodies than conquering territory. (I can’t think of a single war of conquest since China annexed Tibet that has gone well for the invader since WWII.)