What makes you think Illidan stopped being a Demon Hunter?
And that one fact alone is enough to kill the class...Blizzard doesn't need to have two classes cover the same ground. Any story you can tell with a DH, can be told with a Warlock.
You know what it'd take to get those into the game for Warlocks?
Tattoos? Already in via the Betrayer regalia.
Glaives? Give Warlocks Dual wielding and add them to the Glaives.
Blindfold? Create a cloth Blindfold.
In other words...purely cosmetic options that are very easy for Blizzard to add. Or, in the case of tattoos, already have.
The actual class itself is left. Looks are important in many ways...but the looks here are extremely easy to add.
Try this, for example:
http://www.wowhead.com/compare?items...21;45479:4+0:0
Its what you'd get with DW and a blindfold.
EJL