The reason why Demon King army gonna lose in the future is because THEY STICK TO THE MOST CLICHE DEVELOPLMENT.
Rather, I think this series is more of a way to try to "Watsonian Hat" explain what kind of demons would actually create the kind of traps and situations that you see in cliches where the real "Doylsian Hat" explanation is "the artist wanted to show naughty bits". I.E. "What kind of monster would create a trap that only dissolves clothes?"
