IMO, Blizz have just left this open to change as they see fit. "Oblivion" could only be an interpretation from a particular point of view. Unmaking the universe may seem like seeking oblivion to us, but to the old gods it could be something else entirely. They could see the universe itself as a prison.
The difference between Sargeras and the old gods is, Sargeras's goals are purely ideological. Whereas the old gods are self-serving and only care about themselves.
In order for Sargeras to be true to his goals he'd have to remove all traces of the Titans from every world then leave them alone. But there might be other reasons; those worlds might also be a source of power for the Titans or it's impossible to remove all traces of them.
When you consider just how many worlds there are in the universe (literally trillions), destroying a few might be considered a small price to pay if you realise just how truly evil the Titans are. They would not hesitate to reduce the world to atoms if they saw it as necessary.
In the order versus chaos debate, people always see two extremes, when in reality, chaos is the natural state of the universe. Chaos can be a healthy thing especially for reactionary creatures like human beings who are meant to react to their environment. You take that environment away from us and we will stand still and die.
Sargeras could not understand how such truly evil psychopathic beings could exist until he realised that we are all the product of our environment. A lot of people simply choose not to believe that, and that's why change has to be forced. Evil was created by the Titan's vision of order.
It's a shame that Sargeras doesn't find another more peaceful way to achieve his goals, but then, it's an evil universe.