James C. Scott (in "Against the Grain: a deep history of the earliest states [2017]) has a bit of perspective on "collapse":
"Why deplore “collapse,” when the situation it depicts is most often the disaggregation of a complex, fragile, and typically oppressive state into smaller, decentralized fragments? One simple and not entirely superficial reason why collapse is deplored is that it deprives all those scholars and professionals whose mission it has been to document ancient civilizations of the raw materials they require. There are fewer important digs for archaeologists, fewer records and texts for historians, and fewer trinkets—large and small—to fill museum exhibits... Yet there is a strong case to be made that such “vacant” periods represented a bolt for freedom by many state subjects and an improvement in human welfare.
What I wish to challenge here is a rarely examined prejudice that sees population aggregation at the apex of state centers as triumphs of civilization on the one hand, and decentralization into smaller political units on the other, as a breakdown or failure of political order. We should, I believe, aim to “normalize” collapse and see it rather as often inaugurating a periodic and possibly even salutary reformulation of political order."
It stands to reason that Rome's collapse would have been straight away better for everyone, if it had involved a direct devolution of power to local administration; but that is not what happened. There was a long period of conflict and privation that accompanied it, as all the lights went out on the old administration while people were still running power for the new.
To a limited degree, the collapse of the British Empire, gentle though it was, illustrates the same problem: it's not that the new order is worse than the old, but rather that the process of getting there is so disordered and bad.
Was this generally the outcome of the Roman Empire's collapse? Intuitively, I would expect breakdowns in law and order and the collapse of complex supply chain systems (particularly food delivery) to result in mass human suffering, at least in the short term.
The histories I've read of the early middle ages corroborate much of your intuition. Trade collapsed; many luxury goods became impossible to obtain at any price; illiteracy rose; population declined and many citizens died from famine and plague.
But there's often a trade-off between freedom and convenience, then and now. The Roman empire was built upon large-scale slavery and wealth inequality. If you were a member of the elite, then yes, the collapse was terrible; you lost access to all the privileges of the empire. If you were a slave, you probably ended up somewhat better off, becoming either a peasant or a bandit. Not being able to obtain pottery or steel doesn't matter much if you couldn't afford it to begin with, and dying at 40 from starvation is an improvement on dying at 25 in the arena.
> The number of people in slavery would have still decreased
On what basis are you making this assumption? "The fall of Rome" was not an anti-slavery uprising. It was gradual disappearance of the central authority (and all its benefits such as roads, law and order etc).
About 10% of England's population entered in the Domesday Book (1086) were slaves. Compare that with the Roman Empire where slave population (including Rome and all provinces) are estimated at 10-15% of the total.
The biggest buyer of slaves going down would decimate the market, no? I would imagine it would have played exactly like abolition of slavery in Britain or US. There was still slavery across the world, but the number of people that suffered from it went down.
Except the local abbot would happily take 1/10 of the fruits of your labor, the landlord even more, and you stood no chance seeking better fortunes away because roads were in disrepair, infested by robbers and patrolled by mercenaries tasked to kill any fugitive...
But you died of famine and disease more often - population went way down. It became harder to trade even the most basic foodstuff, since simple roads to the closest market became unsafe. Eventually feudalism happened and a lot of simple peasants were literally enslaved to the land.
I seem to recall one essay I read (long ago back in college and which a modicum of google-fu fails to locate) that argued that average caloric intake went sharply up in the territories of the Western Roman Empire in the decades after it's collapse.
The barbarian hordes wandering through weren't tourists. They were taking the food that you needed in order to not starve. And if you tried to stop them, they killed you.
Now, you could (in the abstract) say that a better, more just, and more responsive society emerged. But for those who went through it, there were a bunch of new ways to die.
James C. Scott is a good illustration of 'no skin in the game' syndrome among intellectuals.
It would be very persuasive to see him personally bolting for freedom between warring warlords in present-day Libya, or improving his welfare in lawless mafia-ruled cities of ex-Soviet Union in the 1990s.
But no such chance, so his views may be safely ignored.
"Why deplore “collapse,” when the situation it depicts is most often the disaggregation of a complex, fragile, and typically oppressive state into smaller, decentralized fragments? One simple and not entirely superficial reason why collapse is deplored is that it deprives all those scholars and professionals whose mission it has been to document ancient civilizations of the raw materials they require. There are fewer important digs for archaeologists, fewer records and texts for historians, and fewer trinkets—large and small—to fill museum exhibits... Yet there is a strong case to be made that such “vacant” periods represented a bolt for freedom by many state subjects and an improvement in human welfare.
What I wish to challenge here is a rarely examined prejudice that sees population aggregation at the apex of state centers as triumphs of civilization on the one hand, and decentralization into smaller political units on the other, as a breakdown or failure of political order. We should, I believe, aim to “normalize” collapse and see it rather as often inaugurating a periodic and possibly even salutary reformulation of political order."