Demons in Warcraft are Chaotic and Evil. They want to spread pain and misery and excerpt power over others and fuel powerful magics with the souls of the innocent.
The Titans are Orderly and Good*. They want to excerpt control over the universe in a mechanical and predictable way. They set things on a certain path, and don’t appear to want those things to be mutable.
What Sargeras wants is still a completely predictable immutable universe, just one that is empty. The Burning Legion as a whole has no idea what Sargeras’ end game is (from what I understand). No one is sitting down with rank and file demons laying out ‘the plan’ for the new cosmos and converting them willingly. They are intimidated, fel-infused or killed. Demons follow a ‘might makes right’ philosophy, and the fallen Titan might just be the single mightiest individual in the Warcraft cosmos.
In the end, when every world is burned, and all living creatures serve the Legion, how will the hordes of evil fel creatures react when ordered to fall on their swords and end the last remain lives in the universe (their own)?
No, His goals are completely different than the demons he controls. His methods align with how demons operate, but Sargeras is still doing the same thing he was doing when he was still the Champion of the Pantheon, re-ordering the universe.
Honestly, the Pantheon might not be any better. Their ideal of life might seem like a mechanical parody, devoid of any meaning. They might look at us as horribly corrupt and disgusting, and be just as quick to try and eliminate us in favor something ‘better’.
Titans or Old Gods, these unfathomably powerful creatures are a threat to us no matter what. We can’t afford to live in the shadow of any creature that looks down on us as if it is a god.
*as far as wee know.