Welp, to me you are being a bit forgetful of some points, is that Sargeras was not the only big threat since at least wrath, you had always the baddies revolving around 3 major pillars and we could establish them every expac : Legion, Scourge and Old Gods
Vanilla : A bit of everything
Tbc : Legion
Wolk : Scourge
Cata : Old Gods
MoP : Mainly Old Gods (but it was maybe the least related to any of these 3)
WoD : Legion
Legion : well Legion
Bfa : Old Gods
SL : Scourge (Death stuff)
Df : We are yet to see but you dont need to be some genius to know the Void is going to be playing out.
When you think about it, it is even before always revolving around the 3 big negative power of the cosomology they made during Legion.
And Sargeras was just ONE of these enemies, since Wrath and Yogg-saron there was already some big guesses as to who would win between Sargeras and the Old Gods, and with chronnicles they added the Void Lords above them as the big baddies (which we are yet to see)
To me you sound like these peoples who beleived and said (mostly out of ignorance) that Arthas was the big bad of wow...i dont know but, its like if you told me that Cata was ok bringing up Deathwind or Wrath with LK because "we have Sargeras waiting somewhere anyway"