I have read Glubb's essay and would like to comment on it.
What Glubb writes has been written before , after all the painting series “The course of Empire” ( https://en.wikipedia.org/wiki/The_Course_of_Empire_(paintings) ) date from the 1830's and tell the same thing. Glubb does show a lot of nuance and finesse, but the premise remains the same.
The problem is that since Jopesh Tainter's “The Collapse of Complex Societies” came out, Glubb's reasoning becomes outdated.
My own intepretation of Tainter is the following : complex societies are build upon a promise, in essence that promise is a return on investment for the extra effort.
In a vacuum, it means that the society conquers space, invests in it, then exhaust itself in a search for more rewards - in order to fulfill its promise - then eventually fails, causing people to stop cooperating in a losing game, leading to collapse.
This in itself is already very promising, but incomplete. In history and geography, there is such a thing as “permanence of settlement”, meaning you will find life where nature can host it, and thus areas are always as populated as they've ever been.
Strata upon starta of culture and experience have accumulated in any place since the moment agriculture started there. So the invaders or “new empire” doesn't start in a vacuum, rather it grows on countries already permeated by culture and administration.
When the Arab-Muslim empire arrived in Mesopotamia, they found a functioning society administered by the Eastern Roman Empire, itself a descendant of the Roman Empire, which took over the Hellenistic kingdoms there that stemmed from Alexander the Great's conquests, that swept across a well-administrated Persian Empire, that itself built upon the former civilisations that existed there.
Locally, people don't change that much. Shlomo Sand demonstrated that Palestinians were the former Hebrews, at some point converted to Islam.
So the Empire is mainly a sales pitch for a project. It's the main problem of historians before the 20th century is that they believe that the empire's sales pitch was the historical reality. Look what happens when a project fails or is revealed as vaporware : it's always somebody else's fault. You, yes you, were decadent, and weak, and all these foreigners corrupted us, and this is why our good project failed, you made it fail.
The reason why this sounds a lot like religion (you have sinned and now God is very cross) (pun not intended but kept anyway) is that essentially religion is such a sales pitch.
Sure, the complex society feeds a lot more people than what preceded it, and allows international trade through economies of scale, thus making the goods more affordable. But the society's collapse doesn't mean everybody dies and trade grinds to a halt. The quantities and prices change, but there is still something functioning. If a place craters terribly (like those places obliterated, void of the living in the Thrty Years War) then immediately people from better functioning neighbourhoods come in and take over. No vaccum. Okay, yeah, the Mayan cities. Grant you that one. Or The Inca cities, too. Poor guys.
Cities have always been dangerous places. When it weren't foreign types being the danger, it were locals. Paris in the 19th century attracted the scum from the french provinces and the countryless nobodies, forming dangerous violent gangs like the “Apache”, already contributing to the firearms industry for self-defense (for instance the Bulldog revolver).
Debauchery was already recorded in the friggin Bible (Sodom, Gomorrha and LGBT City, CA) as well as some forms of drug abuse.
All this talk about the glory of civilisation is bending history to fit the narrative. The Eisenhower Years was a time when the civlisation actually delivered on its promises, and thus is hailed as the pinnacle of civilisation in right-wing history books, but it was preceded by the Great Depression civilisational hangover and followed by the Seventies hangover, in which things looked quite grim.
You can find these examples everywhere in the story of any empire. Nero and Caligula ruled in the heyday of the the Roman Empire, but these weren't great times to be there.
It's a cycle of promises and hangovers, until the promises are not believed anymore. The Bush-Senior Era (Reagan Presidency + Bush Sr Vice-presidency, then Bush Sr. Presidency) was followed by the Al Gore Internet Wonder Years (Y2K was just folklore) until the already creeping Cyberdictatorship Years exploded with 9/11. 9/11 was probably the poison that killed the ability to make promises, for good. I see Trump's antics with the JFK files an indication that something has to be done to restore the promise, but this time we're too far gone.
So is it all about broken promises ? No, because humans as social creatures need to coordinate with others and there is always going to be a new promise. Always.
This is how the Eastern Roman Empire/Byzantine Empire lasted 1000 years longer than its Western counterpart : they could rig the whole thing again back to functioning order. The real problem the Byzantines had is they believed false promises from Italians in the Fourth Crusade, who invaded the capital city.
The reason why empires collapse is that they accumulated too much pollution, human as well as physical. Too much humans (usually in the capital city) that are a net loss (they cost more to take care of than they contribute). Too much soil erosion in the countryside (especially in the latifundias that belong to the superrich). Too much rich people to sustain, with too little returns. No reserve in food, ressources and goodwill in case of a catastrophe (so Puerto Rico didn't invent anything :) :) )
At one point, the system can't deliver on its promises because it would have to purge the situation first, and this is something that can't possibly have positive returns, it will always be a money/ressource sink. The former generations could actually deliver because they didn't make that expense...
We are living in such a world. The cheating housewife in suburbia is a much bigger ressource drain than your average Detroit hood on meth. Professional politicans cannot do anything else but lie, for instance on repeating the 9/11 lies, and thus cannot restore confidence. The whole system is unsustainable and bound to fail.
(The worst will be in Chinese cities, though :) )
Where I differ from Jim With Heavenly Hair is that another system may well take over, for much fewer people. The post-collapse world will be full of teenagers eager to learn a new trade. A new system can be set up quite quickly, especially since we've been deprived of bankable promises for almost a generation now. People long for fresh air, and they may fall for the new Empire, once this one has cratered to the ground, taking 90% of the population with it.