You present a really interesting point Jokubas. Especially the bit about the realistic split-off in lore about how the legion would be up against all of Azeroth. Would the Twilight's Hammer ally with the Legion, though? The Old God's biggest desire is to get out of their prisons, and what better way than having a 25,000-year old Archdemon melt off the prison-top with tsunamis of demonic fire?
A lot of different factions would be worrying about a lot of different things. Like how Shaohao was worried about Pandaria during the events leading up to the Sundering. When he heard the waterspeaker's vision, he didn't think, "Oh, we should probably try to go help the Night Elves fix the problem".
It was, "OH *%(&, get us away from this!". What would the responses from the other factions across Azeroth be? Would the Draenei try to commandeer the rest of Tempest Keep and try to rescue who they could on Azeroth? Or forsake their friends and flee the Legion like they'd done for milennia past.