Literally what I'm saying.
If the Alliance is united, then Night elves, Worgen, and the other races would come to the defense of Stormwind.
If the Alliance falls apart, then no one would come to the defense of Stormwind. Just like during the First War, when the kingdom was isolated and alone.
So, as I have been saying since morning (EU timezone), Stormwind cannot be taken if it is defended by a united Alliance, like the one that appears in-game. We have yet to see the Aliance fall apart, like the Horde did, and thus, as it stands now, Stormwind cannot fall.
I never denied that the Kingdom of Stormwind individually, as a single nation would likely fall to a Horde invasion. But, in WoW, we see that the Kingdom of Stormwind is bolstered by an entire Alliance of like-minded races, ensuring its protection.