Malfurion Stormrage yells: What horror is this? A tumor of Nightmare infecting a world tree, here in the reflection of the Titans' own playground?
Malfurion Stormrage yells: If this seed takes hold, the roots of its infestation will wind their way into the heart of Azeroth. The Nightmare would devour our world, claiming everything... everyone. This must not come to pass!
Almost all of the world trees have been dealt with except Hyjal iinm and... Teldrassil.
So Sylvanas was ahead of the curve and several miles away from the rest of the community. If ever there comes a time that Teldrassil will be corrupted by the Nightmare and take note, it never is not invulnerable nor we can conclude we have indeed stopped the true source of Nightmare since N'Zoth is still alive and about to be unleashed upon us with his very own prison break.
Even Andrassil or now Vordrassil which is now a tree stump housing Grizzly Hill Furlborgs manifested a corrupted version of it thereby infesting the Bear Wild God Ursoc.
Could the entity which will be corrupted in that World Tree be no other than Malfurion Himself?!
To wrap things up here's Varimathras line prior to pre-BfA event:
"When your thrones run red with betrayal... when your holy places burn and the shattered mask hangs above your hearth... only then you will know. And it will be too late.
It matters not. You are blind to the true darkness closing in around you."
Red with Betrayal: Delaryn Summermoon and Sira Moonwarden
Holy place burn: Teldrassil, Temple of Elune
Shattered mask hangs above your hearth: Darkshore