Yes, we would be even worse off if we randomly pulled out a superintelligent optimizer out of the space of all possible optimizers. That would, with almost absolute certainty, cause swift human extinction. The current techniques are somewhat better than taking a completely random shot in the dark. However, especially given point No.2, that can be of only very little comfort to us.
All optimizers have at least one utility function. At any given moment in time, an optimizer is behaving in accordance with some utility function. It might not be explicitly representing this utility function, it might not even be aware of the concept of utility functions at all—but at the end of the day, it is behaving in a certain way as opposed to another. It is moving the world towards a particular state, as opposed to another, and there is some utility function that has an optimum in precisely that state. In principle, any object at all can be modeled as having a utility function, even a rock.
Naturally, an optimizer can have not just one, but multiple utility functions. That makes the problem even worse, because then, all of those utility functions need to be aligned.
I took it as self evident that a superintelligent optimizer with a utility function the optimum of which does not contain any humans would put the universe in a state which does not contain any humans. Hence, if such an optimizer is developed, the entire human population will end and there will be no second chances.
One point deserving of being stressed is that this hypothetical super-optimizer would be more incentivized to exterminate humanity in particular than to exterminate (almost) any other existing structure occupying the same volume of space. In other words, the reason that we occupy space that could be re-structured to the target configuration defined by the optimum of the utility function of the super-optimizer, and the reason that we both vitally require, and are ourselves made of resources that are instrumentally valuable (There is hydrogen in the oceans that could be fused. There is an atmosphere that could be burned. There is energy in our very bodies that could be harvested.) are not the only reasons why the super-optimizer would want to kill us. There is another important reason, namely avoiding competition. After all, we would have already demonstrated ourselves to be capable of creating an artificial super-optimizer, and we thus probably could, if allowed to, create another—with a different utility function. The already existing super-optimizer would have very little reason to take that risk.