Story-wise, the Forsaken always have the threat of dying out, since that was part of the point of Sylvanas' whole speech to Garrosh in Silverpine in Cata. And even though they've bolstered their forces a bit with the Kul Tirans in BfA, there is still that threat without Forsaken being created en masse.
However, for gameplay they must always be around, since they're a playable race. So the story will be written to accommodate that.