One of the obvious flaws in comparing them to the US, is that the US power emanates outward from its epic domestic economy, which has been the world's largest since shortly after the Civil War. The US economy didn't require the present size of the US military or its global positioning to get so large - nor did modern Japan, Germany or China (#3, #4, #2) require epic global militaries to do what they've done economically.
What would the downfall be? The US is going to contract physically and lose its overseas land holdings? It has few. It has a massive size core land holding that is the country itself, including all of its resources. The Roman Empire had no such thing, nor did the British Empire, for two prominent examples; their land holdings were largely conquered, foreign cultures. One would have to project the US shattering into state regions, multiple countries. It's very unlikely to be allowed by the Federal Government.
It will be forced to pull in its military and close 100 overseas bases? That will be a benefit, its people will be healthier and wealthier if $250 billion were presently diverted from the military to infrastructure, healthcare or almost anything else domestic.
The dollar will lose its position? The US economy was by far the world's largest prior to the USD reserve standard. If you roll the economy forward 30 or 50 years into a theoretical downfall time, then chop 1/3 off its GDP per capita to represent the downfall -- with 380 million people it would still have the world's #1 or #2 largest economy and still have one of its highest output per capita figures. The Eurozone, Japan and China do not require any unusual global military superpower positioning to have powerful currencies, there's absolutely no reason to think the US does (other than in cases of fantasy US collapse projection); they require large economic output to support and make relevant the currencies in question (Euro, Yen, Yuan).
Right now you can combine Spain and Italy's GDP per capita figures and you are still below where the US is at ($65k). So how far does the US have to fall economically, while having a future ~380 million people, to no longer be a global superpower compared to everyone not named China?
For the US to implode as a global superpower, its economy has to shrink by around 50% to 65% or more. For example here's a % comparison on GDP to other nations over the decades:
In 1980 France had an economy 25% as large as the US; now it's 12.5% as large approximately. Was the US not a superpower in 1980?
In 1980 Italy had an economy 16.6% as large as the US; now it's 9.3% as large.
In 1980 the UK had an economy 19.7% as large as the US; now it's 12.7% as large.
In 1990 and 2000 Japan had an economy 50% as large as the US. Now it's about 24%.
So how far would the US have to fall economically (the base of its military power is its economic output, the ability to tax & fund it), compared to where it was at in 1980 versus other powerful nations, to no longer be considered a global superpower? I think these figures put it into perspective, you'd somehow need the US to massively collapse economically. What would cause that? Federal debt? It can deal with that for a long time yet and ultimately very slowly debase its enormous dollar wealth to deal with it (basically debase the $130 trillion in wealth at 1% per year, so if you're still growing at all it might end up as a net wash or close to it, until you erode the debt enough to flip the scenario).