This story is odd. This isn't how power vacuums work. If something as threatening as a demon Lord is defeated, then either humans or demons would fight for the land or power that would go to the taker. A hero in training would definitely still have work as a mercenary or a soldier.
Even if humans developed a peace to defeat the demon lord, I doubt it would be held together for too long without a demon Lord as a threat to keep that peace.
This world is stupid.
Boobs are big though, so I'll keep reading...