However can you tell me what the difference is between a Demon Hunter using Immolation during Metamorphosis and a Warlock using Immolation (aura) during Metamorphosis? Both are required to be in melee, both are using spells, both are using the same spells. Would you honestly be satisfied with another class getting a simple recolor of your ability? Or your own class (warlock) having their ability recolored?
Since you know demon hunters were the first to use the purple demon form Warlocks now use. This is why from a lore standpoint of course both can exist at the same time. But from a game play standpoint they can't because core demon hunter abilities were given to the Warlock class and used as iconic class/spec defining abilities of the Warlock class.