Yes it could, because it would spread like an infestation and not like an army. And normally there wouldn't be a small army of paladins at hand whenever someone stumbles over a corps.
Also who would make sure that no necromancer or forsaken would use the absense of a proper LK to install himselve as a new one?
Ner'zhul and the Lichking both where somewhat conventional guy's that led a somehow convetional war. That both didn't want a mindless Zombihorde doesn't mean it wouldn't have won. (I personnally still think that deep down Arthas was aware of this and he used this "I let you butcher all my champions to get you as even better champions"-plot to stop himself from overwhelming everyone with a Zombihorde).