Oh, he's wrong about that, but while BFA fails the most in comparison to its expected margin until Vanilla comes along, SL's rate of failure, per the graph is marginally lower and it never reaches the same spot. The statistical defense of BFA, much like any defense of it, is an exercise in clownery. Though mind, the graph is nonsense as well, both because of the lack of the actual required axis and because if we go by this signifier, the worst expansion is for sure Cataclysm, as it dipped the most subscribers relative to where it started.
The clarity of Sargeras as a baddie is that he doesn't appear, making him, intended to be a larger than life divine threat actually work as such, we never find him, and he's referenced (though doesn't appear) for years on end. The fact that until the very end his only presence in Legion physically is as a voice and as a cloud shows this understanding of what the writers and their success at communicating. He's not characterized, but he doesn't need to be, since he's easy to grasp and what he's involved in. Explaining why the Bald Man fails is like explaining why water is wet, but one of the key reasons he fails even as an episodic villain, let alone as Sargeras 2.0 as Steve obviously wanted him to be is because he has no clear concept. Anything from an evil overlord to Jigsaw in space to Skynet to LK 2.0 is, if not a good character, then a character. He's all of these things and none of them and even his visual design is noncommittal dross.