I think if they were all dead, their influence would have disappeared from the world... Which it hasn't.
If Old Gods are physical manifestations of the Void realm trying to grasp the realm of the living, one can say that the same principle of Demons/Wild Gods applies to them and if the veil that separates our world is ever shattered, all Old Gods would return.
interesting, so we might see them again someday
I'd certainly be down for some more old god story if it is spaced out a bit. Using it too many times can get old.
we've learnt many times that dead isn't always dead,
Old gods didnt dead but blizz killed them :) Example: One shot kill released N'zoth :)