I suspect the connotations of “barbarian” are getting in the way here. The Mongols were highly mobile pastoralists and raiders; this did not get in the way of setting up sophisticated and creative institutions. (Nor did the latter undo the considerable net loss in poulation and extent of cultivation that accompanied the Mongol conquests.)
The mongols were emphatically not barbarians, they introduced systems that were in most cases improvements over what they destroyed.
I suspect the connotations of “barbarian” are getting in the way here. The Mongols were highly mobile pastoralists and raiders; this did not get in the way of setting up sophisticated and creative institutions. (Nor did the latter undo the considerable net loss in poulation and extent of cultivation that accompanied the Mongol conquests.)