Illidan is Illidan. Warlocks mimic his powers, but they are still Warlocks. A Necromancer who picks up a sword is not a Death Knight, as we know the Death Knights as former champions of the Scourge. Warlocks who simply use Demon Hunter spells are not Demon Hunters, and the training requires a ritual of permanence that can't be reversed. You can't implement new lore into the Warlock as a spec, that's not how specializations work. Specs are shared and common between all of its class, while Demon Hunters are identified as their own entities.
Demon Hunters use Warglaives and Dual Wield, which is well beyond what Warlocks use. You can't say Hammer/Sword is okay for Paladins to be separate then advocate Warlocks being able to use Warglaives in the next.And no, Paladin as a 4th Priest spec wouldn't make sense because one uses Cloth and a Staff while the other Plate and a Hammer/Sword. At least Demon Hunters wear light armor like a Warlock, they would only need to tweak the visual of Robes on a Demon Hunter to show like a Vest.
Except not everyone is happy. The main people who want to play as a Demon Hunter would know they are just playing a Warlock who can use a Warglaive. If gameplay was the only thing we wanted, then we would have been content playing a Demonology Warlock. We still want Demon Hunters, and not as any part of the Warlock class.In the end this implementation would need some tweaks to keep everyone happy (you don't want a Demon Hunter wearing a robe and a staff for example), but it's still the easiest solution.