I agreee and get where you're coming from
I really wanted one of the Warlock legendaries to be either Kil’Jaeden or Archimonde. The prime Legendary for Warlock has not been revealed yet, but I seriously doubt it’s going to be one of them :( I’m crossing my fingers that it’s not something gimmicky or a ton of randomness or I’ll feel like Blizzard missed a huge opportunity
Honestly they're important enough to be neutral legendaries. Also, isn't the lore of the expansion about Mecha-Jaraxxus trying to revive the Burning Legion? if that's the case that means the original Legion died, and I guess that then Kil'Jaeden and Archimonde went down with it.
Also now that @phryu mentioned George and Karl I imagine them lost in Outland. That could've been fun tbh.