It's inevitable that in a game like WoW the developers will bring back old themes in order to keep the game going. From a story perspective, this is horrible because nothing ever gets resolved.
In TBC we defeated the Illidari and Kil'jaeden's forces. Illidan didn't have to face the consequences of his actions because 8 years down the line he suddenly becomes the chosen one and an army of demon hunters returns from the Nether to back him up. Kil'jaeden doesn't have to face the consequences of his defeat at the Sunwell. The strength of the Burning Legion is in no way diminished since all of a sudden they can respawn infinitely. You could remove TBC from continuity and Legion would still play out the same way, except Illidan would just come back from Outland instead of being revived.
In WotLK we defeated the Scourge and killed Arthas. 8 years down the line people are speculating that we will get a Return of the Lich King expansion or at the very least Bolvar and the Scourge will play a major role in the conflict to come. All of our efforts during WotLK amounted to nothing because the Scourge can still return to its peak power at a moment's notice.
In Cataclysm we wiped out the Black and the Twilight Dragonflights. 6 years later they are popping up in record numbers again (see Island Expeditions). Wrathion's campaign to exterminate all Blacks has now lost of its gravity. The Elemental Planes are fully recovered and have new Elemental Lords. N'Zoth doesn't have to face the consequences of his blundered Hour of Twilight since he only lost one major pawn (Deathwing). He can enslave the reborn dragonflights and the new elemental lords at a moment's notice.
In Mists of Pandaria we defeated Garrosh and dismantled his True Horde. Then time traveling orcs back him up and he gets a 2nd shot at conquering Azeroth.
In Warlords of Draenor we re-defeated one of the Legion's top commanders and foiled their invasion. This doesn't inconvenience them in Legion in the slightest. The fact that we took out Mannoroth and Archimonde is never brought up. Kil'jaeden doesn't have to adjust his plans for the invasion. They just pile in a fuckton of demons and overrun the planet for several months anyway.
In Legion we defeated the Legion. Or did we? Unless Blizzard really hates Sargeras, I don't see him being punished for losing any more than the Kil'jaeden, the Lich King, N'Zoth or Garrosh were. If Blizzard deems a third Burning Legion expansion desirable, Sargeras will just bust himself out of his prison and reform the Legion since we didn't really dismantle it. There was no permanent resolution or closure.
There's no point in defeating all of these hot shots if they just get revived a few years later.