So, as lore gets more complicated, some things don’t make sense. But this honestly didn’t make much sense from the get go. The fact that the Well of Eternity, and the Highborne, has always been described as being an Arcane thing, kind of makes Malygos the biggest moron in the series as a whole.
Let’s start with something simple, Malygos and the Blue Dragonflight easily did their job the worst of all the Dragonflights. Even Deathwing was a better Aspect of Earth than Malygos was Aspect of Magic, because Malygos never did anything. To start off with, I seem to remember in the books he didn’t really know what the Well of Eternity was, which is as ludicroius as Ysera not knowing what the Emerald Dream was. The Well was the most powerful source of Arcane magic, the thing Malygos is supposed to rule over, yet he either didn’t know what it was or at best didn’t watch it at all. The Highborne should never have been able to mess with it without drawing his attention, but somehow they messed with it to the point of drawing Fel entities attention from the twisting nether before the literal embodiment of Arcane Magic on Azeroth noticed anything. Which is ridiculous.
And this isn’t a small thing either, this is why Warcraft happened. The first Legion invasion was what set everything else in the entire series into motion. Even the Old Gods were only able to do most of what they did thanks to the aftermath of the invasion. The Titans decision to make the most antisocial and prideful prick of the five the one who would rule over magic, which practically everyone, even animals, can learn to use in some capacity, is just bizarre. And then in Wrath he complains that its being misused by mortals, who are at least doing something useful with it and not hording it away.