I think she looks so scared because she realized she has fucked up and things will definitely not go her way anymore.
My money is still on N'zoth winning without Azeroth being destroyed. The black empire will rise again.
Speculation: The other old gods are dead, are they in the realm of death? Does Sylvanas maybe has a secret agenda to "recruit" forces for the realm of death to keep the old gods in there? Maybe N'zoth tries to free them or absorb them or something and succeeds in the end? And in the next Expansion we have to fight the old gods with help from death himself?