Quote Originally Posted by Lotharion
The comics confirmed C'thun's death.

People were lead to speculate, as a result of what Blizzard have said regarding the symbiotic crap, that the Titans' method of 'killing' the Old Gods would have killed Azeroth, but we are more small and precise in our killing methods. But again, that's just speculation.
IMHO, this. Maybe the Titans' (speculating here) wanted to rip the Old Gods out of Azeroth, and THAT would have killed Azeroth. Or they could have wanted to blow everything up, when what you just needed to do was punch C'thun in the eye and kick Yogg-Saron's brain, i. e., exploit their weaknesses.
Another thought I had was that maybe they are like Sauron. Their bodies were destroyed, but their spirits (or souls, whatever) continued to exist, in their case living in the whispers and in the insanity they cause.