Do you honestly think a universe the size of ours can only support six billion people before reaching the point of diminishing returns?

That's not my point. The point is people aren't going to be happy if an AI starts making people that are easier to maximize for the sole reason that they're easier to maximize. This will suggest a problem to us by the very virtue that we are discussing hypotheticals where doing so is considered a problem by us.

If you allow it to use the same tools but better, it will be enough. If you don't, it's likely to only try to do things humans would do, on the basis that they're not smart enough to do what they really want done.

You seem to be trying to break the hypothetical assumption on the basis that I have not specified a complete criteria that would prevent an AI from rewiring the human brain. I'm not interested in trying to find a set of rules that would prevent an AI from rewiring human's brain (and I never tried to provide any, that's why it's called an assumption), because I'm not posing that as a solution to the problem. I've made this assumption to try and generate discussion all the problems where it will break down since typically discussion seems to stop at "it will rewire us". Trying to assert "yeah but it would rewire because you haven't strongly specified how it couldn't" really isn't relevant to what I'm asking since I'm trying to get specifically at what it could do besides that.