That it was, but it was just a vision of a potential future. The Void shows all truths but can't distinguish which is the right one.
Regardless I wouldn't mind too much if that vision came to fruition in some way (I'm not expecting Blizzard to just copy-paste that scenario". Alleria needs to showcase her new Void powers (don't forget the absorbed the massive power of a void naaru), what better way to do so than by corrupting the entirety of Stormwind?