This is certainly speculation.

What if we lose at the end of WoD, maybe Archimonde is defeated maybe not, but then the full force of the burning legion is unleashed on alternate Dreanor by Kil'Jaeden or Sargeras, and they are too numerous to possibly defeat, what if we escape back to Original Azeroth just in time, leaving the whole of alternate Dreanor to be engulfed in flame.

That leaves us, back in Azeroth, with the looming threat of OUR version of the burning legion still above us, and an incredibly poignant and brutal example of what will happen to our world if we fail again!

It would also resolve the issue of there being two copies of all the important lore characters... assuming our access to alternate draenor is closed completely

That would be pretty cool...