I don't follow your logic which suggests that Ishamael and Aran'gar have identical goals. Basically, we don't know what the terms of Graendal or Aran'gar's alliance was, so I don't think we can assume she outlived her usefulness to him.

My point is that most of the Forsaken have been defeated or killed by their own weaknesses - Rahvin's pride, Lanfear's over-confidence, Ishamael's hubris, Moghediens' cowardice, for example. Thus it makes sense that a Forsaken would die in an ironic way - ie: pulled down by their own predisposition.