I would posit that, while there are undoubtedly differences between a human as we know it and a monster as we know it, there is no significant 'seperation' to attribute between monsters and non-monsters. They still embody same aspects of what we can consider the human condition. They are still born, they grow both physically, mentally, and emotionally, they possess aspirations in life, the mourn for their dead, they will still run into struggles and conflicts…
I wouldn't necessarily consider a Hero to have removed his humanity, even if he is capable of doing things that humans as we know them are not capable of. Therefore I cannot really conclude that an Incubi is more or less 'human'.Â
Perhaps the outlier would be death. The closing of the book. Even then, I'd consider it unfair to strip away the entire identity of being 'human' over this, even if it is in a sense a step toward godhood. Just as I would consider it unfair to call someone 'less than human' for having superstrength.
So for the sake of what trully determines humanity to Mamono is, well, convenience. Just as much as humans didn't really consider groups each other human for thousands of years, it is easier to put together for people to see remarkable differences between culture, physical appearence, language, etc. and go 'clearly this is not us and therefore it must be something else'. Flesh and blood essentialist stuff is self defeating hoo-ha anyway, it only limits our potential. Did Cro-Magnon emerge from their caves and looked to the stars before saying 'We aren't human.'? Did they not mourn for their dead and hope for good herds to hunt tommorrow as well?Â
As far as MGE goes, especially with the WG3 speaking about the 'what' and Incubi and a Hero is, it definitely feels like KC wants to go to that direction of Monsters and Non-Monsterfolk being one. A good move imo, it at least extended a bone to incubi/humans to ensure they won't be slaves.Â