Iirc, Sargeras created the Burning Legion in order to combat the Old Gods. Azeroth dwellers have long opposed the Burning Legion, and now they (us players) have the Legion cornered and about to be routed once and for all. But wouldn't that imply paving the way for the OGs' rise? After all, the one purpose of the BL is to scour any world whose soul has been infected by the OGs.
Most strikingly, the leadership of the Alliance (the only faction that counts in this xpac so far) appears to be completely oblivious to that possibility, even Tyrande and Malfie, who are both old acquaintances of the Legion, and whose master Cenarius was actually corrupted by an Old Gods' vassal. And he's not the only relevant figure to have gone insane due to OG-scheming; you have also Archbishop Benedictus, Fandral Staghelm and of course, Garrosh. How can anyone not realize that there is something amiss? Old Gods' work at their best?