ts so stupid, he should've had humans in his Antares Empire too. After that, the humans shouldn't have been able to win the war against the Antares Empire because just the king could take out all but one of the hero party. If the king fought the hero party with his four demon kings, he probably would've won easily with multiple strong front-liners stalling the Fist King as the 'demon lord' and his best mages stacked debuffs, curses, poisons, etc. on the Fist King until they could decisively kill him.
Since they had the dwarves, they should've had better gear, technology, and infrastructure than the humans.
Since they had the elves, they should've had better agriculture, medicines, and magic than the humans.
Since they had the orcs and trolls, they should've had much greater brute strength than the humans.
If they had simply invited human immigrants as well, then there would be nothing at all which the human empire should've been better at aside from numbers and being homogenous.. But even the numbers thing doesn't make sense because usually Orcs reproduce a lot, and if they have goblins or even slimes, they should have zero reason to lose from numbers.
Plus, if they had humans living alongside them, the racist human factions would've gotten more cognitive dissonance. "Hey, maybe we can live alongside them after all?"
And finally, if too many of the humans in this story are just racist dumbasses then it loses nuance just to shoehorn in a large group as the bad guys without good reason. If the main character could so easily get along with all the other races, then the other humans in this world shouldn't have that hard of a time doing so either.