Always funny reading how butthurt Old God fanboys are that the simple fact is.................Titans came, they saw, they conquered. Translation: Titans came back to Azeroth to find it infested with Old Gods to the point that killing the OGs would render Azeroth uninhabitable (reference persisting effects of Y'shaarj corruption despite being deaded), so the Titans instead of obliterating the planet (reference Algalon's description of Titan power capabilities) they subdued the OGs, which is far more difficult than outright vanquishing something.
Why is it so hard to understand the Old Gods have no real PHYSICAL power relative to the other colossal powers in the universe. The OGs' power takes the form of corruption. They can seemingly create life (Faceless Ones, etc.) and curse life onto lifeless constructs (Curse of Flesh). Going toe-to-toe in a fist fight is not their style nor their forte.
Lastly, Sargeras has never once in all of millenia been to Azeroth (with the Titans nor without them). And as of his current demonic titan state, he literally has a fel aura that turns everything within miles of him to ash - not very fun times.