>>280334194The demons have successfully taken a warrior who was trained for most of her life to kill them, gave her nothing more than a normal, fulfilling life, and completely psychologically disarmed her in the process.
If the humans ever attack, she'd be more inclined to defend her friends and the demon castle than join back with the humans.
The demon king is probably aiming to do the exact same with the human kingdom, at the very least making them unwilling to fight a war with the demons, if not willing to just surrender with the expectation of a better life under the demons.
After that, even if the demons don't just drop the act and start killing the humans once they're in a position of trust, demons have a massive physical, technological, and cultural advantage over humans.
Within a few generations, it would be a population victory, with the demons now outnumbering and out earning the humans, to the point of population collapse or absorption through race mixing.
The demons win with nothing but good food and manners.