A couple of things about the statements made relating to “empire”.
First, I think this idea of awarding the mantle of “most powerful” or “greatest” empire and or civilization is a fools errand. It’s the same slippery slope one encounters when trying to come up with lists of the “greatest” college football teams ever and the like.
The whole question as to why empires decline discussion is interesting. Most of the time it involves many more factors than those that were laid out and I have a feeling that the ideas presented are largely based on an pretty limited analysis of European empires, specifically the Romans.
Case in point, a book I recently finished called Nomads by Anthony Sattin devles into this idea as to why empires and civilizations come and go. In it he introduces some of the theories Ibn Kahldun who was basically the first person in recorded history to tackle this idea.
Kahldun introduced the idea of circularity as it relates to the rise and fall of empires. The cycle as it was discussed in Sattin’s book and as it was applied to the Nomadic Empires discussed was one of rise, plateau, decline, death and rebirth. The collapse of all empires was inevitable and was perfectly natural.
Now what lead to the decline and collapse? The abuse of the natural systems they relied upon has been a key factor in the collapse of a number of “great” empires.
As to the “decadence” theory, based on my reading of history it’s not the general population writ large that becomes “decadent” it’s the ruling class. Those “decadent” settings however, often lead to great leaps forward in terms of the development of the arts, science, technology and mathematics.
Overextended military? In some cases yes in others no.
The untimely death of a couple of the Khans and cultural traditions surrounding honoring those deaths played a part in some of the various iterations of the Mongol empires declining.
The Black Plague?
Anyhow, I’m of the opinion that one of the biggest flaws in the western view of history that is commonly taught is that history is a tidy linear progression.