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Seems like it would make the most sense from both a support and infrastructure standpoint to end-of-life WoW and come up with a way to transition the existing player base into Titan.
Some would probably argue that many people would be willing to pay subs for both games so why not keep WoW running but I believe that this would greatly impact the sub numbers on both games which is certainly not in Blizz's best interest.
Based on everything I've heard / read, Blizz's plan was to keep WoW going for 12 years which seems to be falling right in line with a 2016 transition to Titan.
On top of that, there are already some in-game stories that suggest the end of Azeroth is upon us (Stranger in a Strange Land quest chain for example).
It seems that it would make sense to build a climatic end to Azeroth as a transition to Titan where a small number of survivors, having somehow escaped Azeroth prior to its demise, find themselves starting over in a completely new world.
Clearly, this is all speculative at this point but I think if we pay attention to what's happening in MoP, we may get some clues as to what Blizzard has in mind down the road.