I’m 90% sure that they originally planned to both rebuild Silvermoon and re-incorporate Eversong/Ghostlands back into mainland EK back in Cataclysm, but then ran short on time and resources and had to shelf those plans.
There’s a fair bit of evidence for this in-game. First, Eversong actually DID get one visible change in Cataclysm: they removed the giant troll gates at Tor’Watha.. This change makes zero sense outside of the context of a larger revamp of the area and aligns perfectly with making the area flyable, since there was nothing behind those gates but mountains.
Second, if you look at the Eastern Kingdoms map behind the Ghostlands instance portal and the Stratholme smoke cloud, you can see where they actually put a small chunk of the Ghostlands onto the EK map, but then stopped when they realized that there’s a spacial collision between Stratholme and Deatholme. Details including screenshots can be found here.