Dark Apotheosis gives them tanking abilities including a taunt and swaps a ranged nuke for a short range attack and is derived from a Glyph known as the Glyph of DEMON HUNTING. Your argument, however, refers to how Warlock tanking USED to be done..draintanking. Drain tanking is something different and while it is possible Blizzard could redevelop warlock tanking around that, its also largely unnecessary to give them two separate tanking styles.
So sure...Blizzard could do that But why bother when all it has to do it tune the existing Warlock into viability instead of redeveloping a new style of tanking. And even were Blizzard to develop it, that wouldn't change the fact that Warlocks are Demon Hunters right now. You'd have to actively remove abilities and options from the class to change that. Which again, isn't impossible but ti does seem unlikely