This just means that species who have emotions generally cannot really coexist and make a stable civilization, this doesn't say anything about their emotional valence and population numbers, which is what the Witch system actually depends on.This is... not actually true. Unless newer installments added these things. Kyubey specifically points out that humanity is an irregularity for the ability to coexist despite having emotions. These emotions are necessary to create energy. Humanity is pretty important to the Incubators because of that.
If there existed countless species with quick generation, short-lifespan/quick-birth and high emotional valence, then they would not one bit be inferior to humanity in energy production, even if they had constant warfare or even worse.
These are two things that are implied to be important, but aren't stated to be so, and from Kyuubey...well.