They share the spotlight as Main Villains.
The Scourge is certainly a major force in the Upper Kingdom, but the Faceless ones and Twilight's Hammer overran the lower depths, as could be seen in WC3 with the Forgotten One and its minions, and in the Ahn'kahet: the Old Kingdom dungeon. The final boss of that dungeon, and the entire WotLK Nerubian storyline, was Herald Volazj, Champion of Yogg-Saron.
No, but it is a dungeon where Old Gods are the unseen overarching antagonists.That trail of thought is so flawed. Is Blackrock Depths a Titan dungeon because the Titans created Earthen who then devolved into Dwarves?
Since the Dark Irons were enslaved by Ragnaros, who himself was a slave of N'Zoth, and Deathwing even sent Twilight's Hammer there to bring the Dark Irons into the Old God fold.
The point is that the Old Gods have burrowed deep underground with their tentacles and creations, so they will feature prominently in Underground expansion.