Every sentient species to turn up in the galaxy that does not evolve to be metapsychic or fails is judged unsuitable for membership in the Milieu by their proctors is quarantined in their home system and doomed to slow extinction as they run out of resources (unless the failed proctorship caused the race in question to wipe themselves out in some kind of war). There have been hundreds of attempted proctorships. The number of species to achieve full membership in the Milieu? Six.
It's strongly implied that once Humanity is fully Unified, the Milieu will have enough power, skill, and empathy to go back and raise all those quarantined species up. This may in fact be the reason that humans are so important in the scheme of things.
FOUR BILLION PEOPLE died in the Metapsychic Rebellion. Well, two billion humans and two billion Krondaku, but the point is the same.