This is so obviously untrue. The stealth games that "define stealth areas, points, volumes, gradients between them, etc" are not fun. e.g. think of grass in an Assassin's Creed game: clearly defined, but the mechanic feels cheap and unrewarding because grass functions as a black hole. Contrast that to a classic stealth game like Chaos Theory, that uses actual visibility and sound, that keeps you immersed and challenged through the whole game.