The jailer doesn't need to exist...besides, after we defeat the jailer we're going to find the plot twist that makes us go "Oh no, he was right all along" and then we'll facing the Arbiter as the final boss....don't think Sylvanas will be the one to take their place but I imagine someone will need to sort the souls of the dead (or we end the expac finding out that the souls don't need to be sorted)