Why, because he said that?
Saurfang's beat him to the threat. Back in WotLK he told Garrosh he'd stop him if he took the Horde down the route it once walked.
So I'd say ol' Saurfang's got priority on it.
The threat was made at the beginning of the Northrend campaign, before he lost his son and became a broken old man. Plus, with the amount of attention being given to Vol'jin, it makes more sense. Still, Saurfang should be involved in the events leading up to and surrounding the siege.
Granted, Saurfang is pretty broken, but even a broken person can do something. He may see taking out Garrosh as the final part of his legacy, seeing as his son is now gone and he has nothing left.
I do think Vol'jin will be involved, but I don't think he'll get the final blow, but we'll see.
Personally, I think Blizzard should do like what they did with Saurfang, Jr. (and, by extension, the airship battle) in ICC, and have two different outcomes depending on the side playing.
Though that would play hell with the canon until we find out who actually did it.