That is exactly right: we cannot know who will be most valuable to the world, and to think otherwise is “playing god.” However, this is a point for side proposition; given that we don’t know who the really valuable people are, we ought to save the greater number because it statistically increases the chances that they will be saved. The only time this would not be true is if the average person had a net negative effect on the world, but if this were the case it would commit us to the implausible position that we ought to act in a manner so that the fewest people survive, which is absurd.