Well the difference between Demon Hunters and Necromancers is that unlike Metamorphosis, Death Knights have had Necromancer abilities since their inception, so the DK player base is pretty attached to them, and its a core theme of their entire class. You can't simply remove a single ability from DKs and be done with it, you're going to have to dismantle multiple aspects of the class in order to keep DKs an interesting class that fulfills their class fantasy, and gives enough material to make a cool and interesting WoW Necromancer. With Demon Hunters, all you really needed was Metamorphosis and some flashy melee abilities.
In order to be a possible class, Necromancers are going to need a lot more than that, and a lot more from Death Knights.