I'm not the same anon so I don't know what point you are referring to.> there's no reason to assume that homogeneous societies in this cartoon world would necessarily be more beneficial than multicultural ones.
There is though.
Or to be more accurate, there is proof that a dominant species run society is more beneficial than one of vast numbers of fragmented multispecies ones.
We had 7 billion humans inhabiting megacities across the world. With advances in education, healthcare, food and relative saftey in a generalized distributed manner.
Ever since the Mushroom war, it's never been the same. Pockets of societies, isolated clutches of city states at best, surrounded and behest by monsters, untamed wildness, and danger on every corner. And every where one turns, ruins and decay.
PB doesn't have her people try to colonize the wildnerness, but shelters them behind the walls of her city. The fire kingdom seems to have little aptitude for expansion either. The Ice Kingdom only has penguins.
As we've seen by 1000 years from now, the Candy Kingdom died out, and that was one of the most powerful societies in Oo.
As I mentioned earlier, the dogs and horses at least appear to have a grasp on a growing civilization building modern cities and infrastructure which is why I am not surprised that if in 1000 years form now they dominate.