Tolkien says this in one of his Letters, forget which one. The Numenoreans at their end were, as far as might goes, very, very powerful. They *could* have defeated the Aman on a purely military level -- even the Valar themselves.
I think the reason the Valar called upon Iluvatar is not only the valid point you raised about killing so many of the Younger Race, but that they couldn't do so delicately enough to avoid probably destroying Aman and many Elves in the process. Witness the results of all their previous battles.
Iluvatar, OTOH, had no such handicap. He can be as widely or as narrowly destructive as He thought best. The action He took in taking care of the Numenorians was direct, powerful, specific, and very effective. Very Godlike, in other words. :)
Tuor
I hadn't thought of that, specifically, but I think you're right.
They didn't exactly seem to be great at subtlety, did they?