No single game has killed WoW, but the secret is that they all collectively chipped away at it. During WoD, retail WoW had 2 million subs and FF14 had about 500,000. Now you can bet that retail WoW has about 1 mlilion, while FF14 still has 500,000. So WoW is not that far ahead of its competition any longer. The only exception is the spikes in players it gets during expansion release.
There was no "WoW killer" but in hindsight they were all WoW killers when combined.