And these are just the opportunities we can take advantage of with existing tech, but the optimizer can also help design new tech.

We can do lots of useful things, sure (this is not a point where we disagree), but they don't add up towards "saving the world". These are just short-term benefits. Technological progress makes it easier to screw stuff up irrecoverably, advanced tech is the enemy. One shouldn't generally advance the tech if distant end-of-the-world is considered important as compared to immediate benefits (this value judgment can well be a real point of disagreement).