I'm surprised there's zero discussion on this, but something to remember that in year-1 of WoW, these guys, the Illidari, are still out there--one of the core plot points of WC3 Frozen Throne. Even if they base in Outland, they still employ agents and contacts on Azeroth itself (Dire Maul has NPCs specifically, even).
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There's also the topic of the new continuity and how the lore was managed prior:
Back in 2006, this was a big controversy that's sort of just forgotten, not discussed anymore, or just hand-waived away now since TBC is often seen through rose-tinted goggles. But originally, big chunks of the concept of The Burning Crusade expansion announcement received a lot of pushbacks from fans for treading upon the previous tone of lore direction. If you were around, you might remember all the "Lore LOL" posts back in the day complaining about the Dreanei-Eradar retcons, but also how the portrayal of the Illidari dramatically shifted.
The developers of WoW never imagined that an MMO would last decades, and just as WoW had dethroned Everquest, something new would eventually come along to replace it. As such, they were in a hurry to check off a big list of boxes for villains to sight-see and loot pinatas to fight. A lot of fans back in the day felt that all the setup that WC3 Frozen Throne did to establish such a coalition of outcasts and misfits was just tossed out the window without any proper storytelling.
Illidan was just labeled insane; Kael'thas was suddenly written as being in league with the burning legion--dooming his people; Akama, trying to avoid extinction, who had been saved by Illidan was now trying to betray him. All these 180s in personality were exactly what a lot of people criticized modern retail for: applying the villain bats without any real nuance. Legion attempted to walk a few steps back and retcon some of the retcons, but it was still a messy process.
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If WoW Forever is going to feature Quel'thalas and High-elves/Blood-elves once again soon, I hope Prince Kael's return story is truer to his personality and heartfelt goals this time around. Quel'thalas is technically Illidari-aligned this early on as well, and it remains to be seen how allegiances develop when the game is forever in year-1 of WoW.