I love Old God Lore.
But I genuinely think that N'zoth will be the last one we kill.
You guys also need to keep this in mind:
Just because the Titans couldn't kill ALL the Old Gods, doesn't mean they couldn't kill SOME of them.
The ones they killed probabaly weren't completely rooted into Azeroth and tied to its very being.
Because of that, the most dangerous were locked up and sealed.