Are they truly immortal?
I mean, I'm just thinking here, but the Titans didn't kill them because i'm assuming they are similar to a cancer on the planet, hurting them will in turn hurt the planet. But that doesn't mean theyre immortal. Like anything else, overtime these things lose hold as it is apparent they have.
Okay okay, sure, you can listen to the faceless ones (biased as it gets when it comes to Old God fans) who told you that you can't truly kill an Old God. OR...you could think about it this way. The Old Gods are able to be destroyed, and it was the link between the curse of flesh and the Old Gods which prevented the Titans from destroying the Old Gods. The Titans must have thought that the Old Gods were acting with some symbiotic sort of relationship and killing them would have doomed the races of Azeroth, and maybe the planet itself.
As far as the player knows and from the player can see. C'thun is dead. Yogg-Saron is dead. Ysaarj is dead. We have no indication in game that these entities will ever revive on their own terms. As the players end Yogg/C'thun, we see nothing to tell us that they are anything but dead, nothing of course aside from the mindless banter of the spawns of the old gods themselves. (Which, if you were trying to be threatening, you of course would douse yourself in mystery.)
The titans have been wrong before, let's stop jumping to conclusions that the Old Gods are immortal based upon what Titans have told us, given that in their infinite power, they still are not perfect. The Titan's are advanced far beyond any of the races of Azeroth, but that does not mean that they truly understand the nature of every malevolent entity they encounter in their quest for order.