First, I'd just like to say that the story established something which was very clear to me: Sylvanas is the motivation of the Forsaken. Without her, they'll follow Garrosh or anyone quickly. They have an incredibly strong will... and I guess "purpose" would be a better word. But with Arthas dead, they need a purpose. Sylvanas's purpose is to not die. The purpose she gives the Forsaken is claiming their place in the world. Without her, they would surely die... as we were shown.

But I also have to point out that none of your comparisons are relevant. This isn't like the Broken, who got cut off from the light but could work with the elemental spirits. Nor is it like Bridenbrad, who was plagued but not yet undead. These people died. In some way, their souls rejoined the Light. What's left... is a shadow of that person who had nothing to live for but revenge. Necromancy perverts both the body and the soul.

Just look at the Scourge. If uncontrolled by the Lich King, they would become an unstoppable force of destruction. But when the Forsaken broke free, Sylvanas was the one who took control. I see "Lich Queen" used as a title for her often... Maybe that's true, but she resembles Bolvar, not Arthas. She's channeling all the destructive power of the Forsaken and using it at her own will. If one of those many Light-wielding lazyasses were to go help her find a kinder purpose in unlife, she could get the Forsaken to do it.