I don't think it's right to discredit Nadal even if he IS quite one dimensional. He does it better than anyone else in the world.
Also, Nadal has improved A LOT, and I mean A LOT as his career progresses. His added MPHs to his serve, his forehand got MUCH more aggressive (except when compared to early 2004, when he often went for broke off that wing) and reliable, and his backhand has turned from a liability to an occasional weapon. If he is declining, it's due to his mileage, not his lack of skill.