Exactly.
The Undead Campaign of WC3 shows the control of the Nathrezim and Arthas breaking. Sylvanas takes command not through mind-control, but leadership, and as the last of the Nathrezim died, the Scourge was unleashed. Arthas contained them, but left the Forsaken free because they were too far/many/whatever.
The Forsaken in Vanilla through Wrath were all that there were left in the world. Every one of them was irreplaceable, and their deaths represented the waning of their whole race. When the Valkyr were able to create new ones, it was the Forsaken's way of "Reproducing". But Sylvanas wasn't cool with the whole Mind-Control thing, so everyone got a choice.
The first encounter you have as a Forsaken in Cataclysm is a Valkyr telling you that you have 3 choices.
1) Become a Forsaken and Serve Sylvanas.
2) Become a Forsaken and Leave and enjoy your new unlife.
3) Go back to being dead.