This is essentially incorrect. While we can point to legendary incidents such as Carthage in the history of empires, the Mongols are notable in premodern history for systemically using this as a strategy to conquer large rivals by fear and hold a territory size that conventional pre-modern empire building would not allow. Regardless of morality, most empires were better positioned to preserve and exploit life and property than the Mongols. Archeological evidence supports this. This kind of conquest isn’t right either and we aren’t saying all non-Mongols aren’t evil (and I appreciate you were also trying to make this point.)

Total war is a separate concept from genocide. Though both can be practiced in the same war, targeting economic and political support for war does not necessarily mean systemically eliminating a population.