Thanks for the interesting topic.
One thing to keep in mind is that historical empire examples took hundreds of years to decline and fall.
Rome's decline is typically sited as 235-476 from the Age of Chaos to the Sack
The British Empire is in visible decline from within but avoided their sack in WWII but it and its formal colonies are in full decline. This is not a particular dig as Oz, I grew up in Canada and if anything it is doing worse than here or NZ.
Empires can of course make comebacks if they hang onto their identity/territory and gain a strong leader/challenge - China, Russia (USSR) all made comebacks with totalitarian socialism and even Rome avoided an early end when ruled and reformed by Marcus Aurelius.
Can the American Empire find its way through its obvious current troubles? (unfortunately Trump is no Aurelius and Obama was a Chamberlain clone) - seems unlikely since long time fundamental problems - race, urban/rural and wealth imbalance are spiraling.
Personally I believe the colonizers of Mars and the Asteroids are the next empire and Earth will function as a 'yesterday' place like Europe does to NA. Which cultural identify group will make the sacrifice here to fund/support the leap to the solar system? Here is the chance at 'greatness' but I think we are too busy with more 'important' issues (gotta mail that vote today).
