The less they explain the better it will be for everybody, just go with the flow and don't look back.
If you just don't want to give up and do what I said before then maybe you could answer me this, considering that this is a multiverse with infinite versions of everything contained in them, then how odd would it be to have an alternate universe where Sargeras never surrendered to despair and the pantheon is still alive?, how odd would it be to have said alternate Sargeras corrupted by main universe demons?...
This is why you don't mess with multiple universes, you're just giving your masterpiece an infinity of weak spots, it is even easier to answer questions from your wife when you get home at 6 am drunk and with a bra hanging from your pocket.