Today, it is not completely clear that Darwinism really describes how evolution functions, although some of what Darwin observed seems logical. There is a counter argument that suggests that humans may function much more often on a cooperative than a competitive basis. This debate would need to be resolved prior to defining any kind of world policy.
The world should not adopt a Darwinist policy. It would be too messy and too destructive overall. To say that one group, country, ethnicity, or certain type of people is "better" or "worse" is just a mistake and would have disastrous consequences. It might result in a self-fulfilling prophecy, but is that really necessary?
This, in theory, is what drives nature over time, but using it in a manipulative sort is wrong. You can even go as far as saying this method is equitable to what Hitler was doing. He felt the Aryan race was superior, and the Jewish people inferior. He felt the Jewish people needed to be taken out because only the "strongest" should survive. It's a eat or be eaten mentality.
This world already does have an underlying survival of the fittest rationale, only it doesn't describe itself as such. We talk about caring for weaker nations and vulnerable people in our own society or others, but it truly is largely talk. It is like the story that covers up the reality, because if people knew the reality they would revolt.