The number of people in slavery would have still decreased. The fact that it continued to exist is immaterial to the point being made.
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.