The United States, as the richest country in 1944 used the Bretton Woods Accords to buy a counter-USSR alliance. The deal was, we open our markets, protect international blue water trade on our dime so long as we get to call the shots. The reason for that alliance was over when the USSR collapsed thirty years ago, but we were stuck with overwatch and involvement in the Persian Gulf as we needed that oil, too.

Now the US is a net petro exporter and there's no real need to spend resources on a global overwatch. North America still has the geography that made it rich in the 40s. There's a short-term issue with demographics, but the Boomer generation did have kids unlike many other countries.

As far as why the Roman Empire fell, you seem to be proposing a variation of Gibbon's thesis only you're using decadence in place of Christianity. It was neither of those things. The beginning of the collapse came from the rise of a professional soldier class (Roman yeomen could no longer fight battles and be home in time to do critical work on the farm), the use of non-Romans as soldiers, and a mix of soil exhaustion and disease. Once the Empire started to contract, Rome no longer had control of the mines it had expanded to control - no mines meant currency contraction making it difficult to pay those solders (fiat currency wouldn't be a 'thing' for 2000 years or so). The Eastern Empire (the half with more resources) continued for another thousand years, and nearly conquered back the remains of the Western Empire - but that was interrupted by the bubonic plague (which is carried by fleas, not decadence).

All that said, there's a surface parallel between the Roman Empire and the American 'Empire', mostly in the "single country makes security decisions", but the similarity ends there. The Romans, when they acquired an ally (either through conquest or on the prospective ally's request) became enemies with all that new ally's enemies. The US basically bought an alliance (Bretton Woods, above).

TL;DR: yeah... no.