That's less a contradiction and more an evolution of her character within the narrative. She once did wish to secure the future of the Forsaken (for her own ends mostly), but now that she's in good with a proverbial god who can control life and death she no longer needs the Forsaken, and no longer needs to pretend she cares about their plight depending on how charitable you want to be toward Sylvanas' character. It doesn't apply anymore, but it's not a contradiction either.