See, this is the problem with you. You refuse to entertain the possibilities of mechanical differentiation, claim to know what rules, if any, that Blizzard follows to create new classes, you flop back and forth between magic schools mattering and not mattering, attempt to disingenuously pad lists, etc.
Necromancers can utilize their minions in ways warlocks can't. Comparing DK diseases to hypothetical Necromancers is like suggesting that Rogues and Feral Druids can't exist because Warriors already have rend, etc.
The exact same arguments your using against Necromancer, you used against DH. Where exactly did that get you? Unlike Demon Hunters, Necromancers have far more room for growth and differentiation, DHs were cemented before they were considered for a class.