But the Pantheon only fought the Old Gods that are now imprisoned on Azeroth, and we do have sources that there are more Old Gods (for example the one that is shown by some Arrakoa was stated to not be one of the Old Gods on Azeroth), so with all Old Gods combined, who knows what would happen. The titans haven't shown much of themselves anyways, and they are aware that something's going wrong on Azeroth, else they wouldn't have send Algalon. But if they could deal with the Old Gods (to re-imprisson them), I don't see why they wouldn't just do it. So I guess they either have something else to do (that is more important than saving one of their planets) or they do fear the power of the Old Gods.