It's just taking history a while to write it down
Varys said "power resides where men believe it resides." That's why Bin Laden attacked the World Trade Center, the Pentagon, and narrowly missed the Capitol. He attacked American financial, military, and political power right where it lives. And two out of three, he hit.
20 years after 9/11, the Americans finished the job. Trump hit what Bin Laden missed, inciting an attack on the Capitol. His mob killed and burnt their way inside, even smearing shit on the floor. This wasn't an external attack anymore. The barbarians were inside the gates.
External attack, internal revolt, over-extended military? We've seen this one before. This is just how empires fall. You can't see history when you're in it, but it's already been made. The date is set, though the words are yet to come. The American Empire fell on September 11th, 2001. As Varys didn't say, "stick a fork in it, it's done."
People In History Don't Know History
People living within empire often cannot perceive A) that they're an empire and B) when that empire falls. We all live in the American empire (almost every country on Earth has American troops in it, or dollar-denominated loans) and that empire has already fallen down.
The closest parallel to American collapse is the Roman Empire they modelled themselves after. The fall of Rome was equally imperceptible to the people living at the time. As Eleanor Janega writes "If you asked a Roman on the street what they thought about the fall, they probably wouldn't understand." Some dude deposed a dude in the year 476, but at that point the eastern empire had been freelancing for years, provinces had fallen off, the empire was already fucked. That year was just a symbolic point in a continous decline.
In the same way America has been fucking itself since the 1980s and 9/11 was just the symbolic point when everyone saw that the empire has no clothes. You can't have your temples desecrated, army defeated, and then just keep going. You might still have power, but without the belief in power, it's just a matter of time.
The symbolic hit on the residence of power is matched by the evidence of troubles to come. America's zombie financial system is going to crash, their military has already lost multiple times, and their political system is about to implode into full fascism. I guess you could date the fall to any of those points, or to the more obvious point when everything caught fire. Which was 9/11. Historically speaking that was the end.
It's Not Revenge If You Lose
Presumably, however, America could have recovered from 9/11 if they'd kicked ass afterwards, but they just killed millions of random people and punched themselves in the dick. It wasn't the killing and torturing people that was the problem (ask any empire). It's the fact that they did all that and lost.
America spent decades and $21 trillion dollars murdering and torturing random Muslims in revenge, and then had to beat an ignomious retreat.
America spent a staggering $21 trillion on post-9/11 militarization. They bombed the shit out of any random Muslims they could find and just lost pallet loads of billions of dollars in cash. They created self-occupation agencies like the Department of Homeland Security and ICE. And all for no gain. America is the only empire that attacked other places and mainly looted itself.
While arms dealers made trillions and their media made billions, average Americans live like shit. For that amount of treasure America could have decarbonized its entire grid, forgiven all student debt, guaranteed preschool, or created millions of jobs at home. They could have gone to fucking Mars. Instead they made the poorest places on Earth poorer and arms dealers richer.
In the end things played out just like Bin Laden predicted. As he wrote in his Letter To The Americans:
America went on a Crusade after 9/11 and got sent back as suckers and losers. The country was mortally wounded on 9/11 and while it could lash out to kill nearly a million random Muslims, it was not strong enough to alter its fate. They tried to get revenge and just lost.
And so it ends. America was the lone superpower for barely 10 years. The American century lasted exactly 9 months, even less than the last Roman Emperor (10 months). They got hit once in 2001 and flailed about in a blind rage for 20 years, just making things worse. Though they're still strutting around the world stage, America is just a chicken with its head cut off. Historically speaking, they're well and truly plucked.