this AI which does absolutely nothing in our world is no more useful than AI that does nothing in all possible worlds, or just a brick.
Sorry, I wasn't being clear. The point was saying that no AI can do better than humanity implies that our world is optimal out of all similar worlds. (I believe there are much stronger arguments than this against what you are saying, but this one should suffice)
It only implies so if your AI is totally omniscient.
edit: Anyhow, I can of course think of AI that can do better than humanity: the AI sits inside Jupiter, and nudges away any incoming comets and asteroids, and that's it (then as sun burns up then burns out, moves Earth around). The problem starts when you make the AI discriminate between very similar worlds. edit: and even that asteroid stopping AI may be a straitjacket to intelligent life as it may be that the mankind is a wrong thing entirely, and should be permitted to kill itself, and then the meteorite impacts should be allowed so that ants get a chance.
I laughed: SMBC comic.