I’d put it a little differently. The original stories of Sylvanas and the Forsaken were one and the same: get revenge on the person who did this to them—Arthas.
And then Arthas fell, and neither Sylvanas nor the Forsaken played a significant part in that (which could have been a building block for future story if they had). And then the writers had to scramble to figure out what to do with them. And that was around the time they started sliding more and more into tying the stories of various races to their racial leaders, a trend that has increased with every expansion since. So their story got folded into hers, but it didn’t have to be that way.
That’s also around the time Metzen started being less involved, and I think Reallyhappy may be on to something with the suggestion that the other writers didn’t actually enjoy writing the Forsaken very much (see this post).