Every fallen empire in history could have reversed its decline.
Rome could've reversed its decline, even into the 400s.
The French monarchy could've reversed its decline in the early 1780s.
The Soviet Union could've reversed its decline as late as the mid '80s.
The problem is that the actions needed to reverse that decline are not the ones taken because of that degeneration fostering weaker & more selfish men taking power. Certain events eventually push an empire past its Rubicon, and it's usually one event that feeds into/creates multiple new factors that ultimately spiral out of control.
Sometimes an empire gets lucky and postpones their decline, even entering a new golden age if they have the right people taking charge. But this is one of the more rare political events out there.
The USA as a world power almost had it all. No one can seriously deny the USA is a great nation that has done great things and that our peak was between 1945 and 1980. But are we all that we could be? Absolutely not. We're a country with so many sins (that we get angry about whenever someone mentions) that it does sometimes feel like it's more of a case of "we're the country that was in the right place at the right time" rather than anything more than that. We're the country that emerged from WWI & II unscathed, have a market economy that has been roaring for 150 years without much interruption, and generally have a hyper-hedonistic culture that celebrates overconsumption. The first point, more than any other, is the one that's responsible for our meteoric rise. The second one is the reason why we've risen so far— the Old World and even South America have been through many wars in the past 150 years since the American Civil War, whereas the USA has seen near absolute stability outside of native American skirmishes and Pearl Harbor). And the third one is why it's so attractive to come to America: there's no societal mores to live within your means or to live by honor & family or to sacrifice for the mother/fatherland— if you want to get so fat that your folds of fat fuse into your couch, you can and you're almost rewarded for doing so. America is a very hedonistic land in a very secular time.
I think we're doing ourselves in due to some unresolved sins, such as racial politics and an eviscerated labor movement. In terms of racial politics, you can point to the Civil War and Reconstruction in the 1860s & '70s as being the root cause for that. My hypothesis is that the US South is so racist because we never had the chance to properly reform ourselves. Slavery was forcefully unimposed from us at the worst possible time: well after it had become part of Southern culture but before it became unprofitable (like in Brazil). If slavery had been abolished in the 1790s, the South wouldn't have developed such a regressive and divisive culture. But if slavery were allowed to continue, it would've eventually caved in. As the industrial revolution progressed, it was inevitable that slavery would've been abolished sometime in the late 1800s or early 1900s, because otherwise the South wouldn't have been able to catch up to the North. The process of abolishing slavery organically would've led to protest movements and a civil rights movement (obviously a very bloody one), but one that would've also fostered cultural reform. Jim Crow possibly wouldn't have arisen.
But the Civil War did happen, and then Reconstruction did, eventually giving us the Gilded Age. Reconstruction ended at least 30 years too soon. The South was given free reign to practice Jim Crow, bringing back racial politics and creating something of a caste system— something that actually arose due to the early labor movement's gains. In an attempt to break the labor movement, business elites tried dividing the workers along racial lines— so whites were superior to blacks and Asians, and thus their labor demands had more sway (which is: zero-plus-one to the elites). And here you saw the rise of such enforced racism, something that eventually bubbled over into wider society and caused a lot of fucky things (including when post-racial social justice politics started rising, which almost always value & promote non-WASP culture), and ultimately led to a deep dividing of the American character. Who is a "real American"? That depends... But you always hear of hyphenated-Americans these days. But for whatever reason, bringing up events that happened within the lifetime of some people is being overly sensitive, that festering issues don't actually exist— and that efforts to rectify them that actually make the whole discussion worse are in some way 'progressive' and shouldn't be debated upon.
Similarly, the labor movement had once been able to keep American society together and our workers well-fed and satisfied without the need for socialism. We had ample money for public works and societal focus on keeping things stable because of it.
Then we started slashing welfare & public works as well as undermining unions, all the while breaking apart the syndicalist movement. It's at the point now where Americans are hostile to the very phrase "working class", aka the base that keeps the entire thing going. If not for the great gains wrought by the digital revolution, the increase in our quality of life would've stagnated in the 1980s, and we'd probably be much more polarized than we already are.
With no ideological rival to challenge us, all of those worker protections are being rolled back and then associated with radical feminist & social justice talking points to keep the masses of society away from bringing them back (which can backfire if the masses of society still agree with radical feminist & social justice types or feel they're allies).
We aren't interested in spreading republican democracy as much as we are American hegemony, even if that means establishing dictatorships or funding absolutist monarchies. We aren't interested in great collective sacrifice for even greater works (like establishing a lunar base or a Great Society) and are leaving that up to the market— which is fickle and imperfect and subject to change with the whims of consumers. If the market sways next year to bring down SpaceX & Blue Origin and commercial space flight becomes unprofitable for generations, there's nothing we can do about it because we've decided to go all in on privatization. Or at least, we've elected people who are financially incentivized to go all in on privatization to maximize their shareholders' profits. We are too apathetic to care. Any discussion of protest is shot down as being incapable of achieving anything. Talk of rebellion in the media is purely to stand against existing social order with only slogans and vague ideals in the way of "what we're fighting for." Our social safety nets are antiquated and complex, and in need of great reform, reform they won't get because they're supposed to be antiquated and complex. We are obsessed with punishment and violence and treat things like the STEM field or creatives as an "other"— real Americana is football, military gunwank, and hunting; engineers, roboticists, and artists are just nerds and intellectuals. Everyone wants to be in their own group, and there's no real dominant social group anymore because of it. The ethnic majority is divided amongst itself but also divided against ethnic minorities who are growing in number, ethnic minorities who are being told to not assimilate or aren't considered to be "true" Americans in the first place and thus can't assimilate. We're obsessed with our military, using it to promote our interests worldwide while simultaneously angsting about being the world police force— even though the major threats to our rule are nations that we can't militarily engage against without triggering nuclear war.
We fought a "war" on drugs for decades, incarcerating millions (more than the number of people in the Soviet GULAG), primarily ethnic minorities, which destroyed societal cohesion amongst a large subset of society and fostered the development of useless drugs that have only increased anxiety.
We are obsessed with the Constitution, particularly the second amendment over all others, and are unable to properly discuss these matters civilly.
Our society is both extremely open as well as very puritanical: we sell sex overwhelmingly, but we're also disgusted by it. We're fine with ferocious ultraviolence in media, but showing a single female nipple is considered to be corrupting to the youth in some vague manner.
This and so many others are causing a lot of societal anxiety in America, but it's not an existential breakdown. Yet. The decline truly began in 2003 when we squandered our post-9/11 goodwill and invaded Iraq, thus triggering us to waste $6 trillion on a pointless war. Obama delayed that decline somewhat, but it was back in full swing easily by 2013 or 2014. Trump has accelerated it extraordinarily— geopolitical goodwill that Obama could only barely scrape back is being lost the world over, and our trade war with China is being fought in the worst way possible. If we lose (i.e. if a depression strikes us and China's able to force a trade deal on their terms), god help us. More than anything, if we lose a military excursion (say, against Iran or Turkey or, god forbid, North Korea), the American psyche will be bent over and dealt a fat one. One of the big deals about being an American is that we've never "lost" a war. Sure, we'll admit to losing Vietnam, but there are still people who contend that was actually a sabotaged victory or point out that America won most of our military engagements and only "lost" because North Vietnam invaded the South after we had left. We don't consider the War of 1812 a loss because so few even know about it as well as purely because of the Battle of New Orleans (which happened after the war was already over). So we consider those two to be a tie, and then point to us being "Back to Back World War Champs" to drive our honor of us being invincible.
What we need to be dealt a true societal blow is a defeat. No wishy washy "we wasted trillions of dollars and lost thousands of lives for an unstable peace and continued occupation" peace a la Iraq or Afghanistan or a stalemate like in Korea, but an actual defeat. Like Russia vs. Japan in 1905. Something where we're completely routed and have to surrender or sign a peace treaty on our opponent's terms. There's no way to spin it: we've lost.
It's really hard to communicate to foreigners how deeply Americans care about our presumed invincibility, precisely because America is such a young nation and hasn't had the chance to lose many wars. For England or China, 1776-1789 is practically modern history, but for Americans, everything before 1945 might as well be medieval times. In our time of being a nation, we haven't formally lost a major war (again, Vietnam is still contentious even though anyone else would consider it a loss). Americans have let this go to our heads, even though every great empire in history has lost at least one major war. Usually more. If you want to break America, let us lose a war.
Then you'll see a mass sense of decline.