As for the OP's premise, Sargeras' goal is to undo the Titans' works. One work of the Titans was to chain the Old Gods. Cutting them loose and letting them return the planet to its hellish pre-Titan state sounds very much like something Sargeras would do.
Power in the WoW Universe is Titans > Old Gods > Legion
Sargeras going through the portal would've freed the Old Gods, not intentionally. They would've probably been all too happy to annihilate the pathetic Legion of usurpers, all the night elves and dragons, and because we know the Titans had systems in place to deal with such an event, said systems would trigger the self-destruct on Azeroth and re-origination.
If it's not an elf, leave it on the shelf.
Night Elf Liberation Front N.E.L.F.
Something mean about trolls.
I don't see any people going to jails open the cages and then begging for mercy, that would be stupid thing to do.
Specifically when it talks about the titans vs the old gods it only states about one Titan that "fell" during that fight. It doesn't state who it was or what happened to it/him/her.
Sargeras left the pantheon and then fell into despair separately.
Med'an woke up in Ahn'Qiraj to the dulcet tones of Cho'gall, who was berating Stasia for her failure in bringing Garona back, to Stasia's protests. After Stasia admitted she was just trying to get Garona killed, Med'an tried to attack them both and got himself flung across the room for his troubles - landing right on top of the corpse of C'thun. Despite being a corpse, the thing immediately starting whispering sweet nothings of chaos and destruction in Med'an's horrified ears.
I cba looking for a quote for it but, it has been stated before that Titans did indeed die while fighting the Old Gods and their minions. People tend to think there is the Titans of the Pantheon and that's it. But there were in fact many more Titans plus their creations and so on. You think only one Titan possibly died while fighting the super powerful Old Gods?
For Hitei, yes you are correct about Cthun not truly being dead even when we killed him in vanilla. Old gods are strange beasts.
Ah heres my source (one thats in game I do believe)
In the time before time, when the world was still in its infancy, a battle between a Titan and a being of unimaginable evil and power raged on this very soil. The prophecy is unclear about whether or not the Titan was vanquished ... but it illustrates that a Titan fell.
Sargeras is Lawful Evil
Old Gods are Chaotic Evil
they don't play for the same team.
For all we know, Old gods can't die at all. Their 'corpses' could always be revived.