> 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.
Slavery continued to exist in Western Europe after the fall of Roman empire until at least year 1000 AD.
In other former parts of Roman Empire such as Northern Africa slavery continued to flourish almost until the 19th century.