Yeah, but the Old Gods are also much less powerful than Sargeras himself. I doubt N'Zoth is going to be friendly about it with the players, if it happens, but it could boil down to - "Sargeras is too powerful for either of us to defeat alone and will definitely kill us both unless we team up and defeat him. Afterwards, we can go back to killing each other."
Together we might have a chance to stop the Legion. Afterwards, well, we've killed Old Gods before. I think we have a chance at killing N'Zoth too. Although doubtlessly N'Zoth would have something cunning up its... tentacle.
Well the three Old Gods who were alive then wanted to basically hijack the portal to escape themselves - reprogramming it so that it would let them out of their various prisons, rather than acting as a way to bring Sargeras to Azeroth.
Presumably they intended to then shut down the portal so Sargeras couldn't use it at all, and then get on with corrupting Azeroth.