Azshara has certainly been confirmed to be a new threat, long hidden since ancient times, finally emerging.
Could it be Azshara, not Sylvannas, lights up Teldrassil? I mean, it's the perfect crime for her -- she gets a war for free. The lighting of Teldrassil is also thought to be the first of five 'torches' which will herald the coming of N'Zoth.
Did Sylvannas really intend to occupy Darnassus or even think she could keep it while the nightelven army remains undefeated?
So better question - why would Azshara want to wipe out the Horde? It makes sense why the battle at Undercity is so dire and dire actions are taken. It makes sense why the Zandalari are the Horde's salvation...
I think it could be that we've given Sylvannas too much credit. She got greedy with the Azerite, thought she had the initiative and got burned, a blunder into an unforseen betrayal that has now both shifted the balance of power and awoken the full ire of the Alliance.