What if after the decline of the Roman Empire, feudalism became so entrenched (let's call this version "manorialism") that individual manors were actually split into demesnes, vassal dependents, and patches of ecclesiastical land that were farmed by helots?
Instead of feudal planters seasonally subletting their land to transhumant shepherds and migratory free peasants who could terminate the lease at will, the peasants would actually be attached to the land directly by hereditary tenure and forbidden from leaving, property of the lords. Arguably, this happened in Venetian-ruled Illyria, but nowhere else was stratification that steep.
If this "manorial" system became widespread, would Europe be stuck in a "Dark Age" for centuries?