I think the problem is that optimization is AI-complete. Without a lot of context about what your program is doing under what circumstances the problem is not solvable. You need to know when and how a specific code path is run.
Agreed, I guess that when we have build smarter compilers a Centaur approach would work best (like in chess). The computer can do a whole lot by bruteforce and smart algorithms and the human uses his knowledge of the context to steer it in the right direction.
