Rather, you were saying: If the AI achieves the goal, it will want nothing further, and therefore automatically act as if it were shut down.

If you don't provide an explicit shutdown goal (as Dorikka did have in mind), then you get into a situation where all remaining potential utility gains come from skeptical scenarios where the upper bound hasn't actually been achieved, so the AI devotes all available resources to making ever more sure that there are no Cartesian demons deceiving it. (Also, depending on its implicit ontology, maybe to making sure time travelers can't undo its success, or other things like that.)