Alsha strength comes from people belief, faith in "hero" but it has to be The Hero, Mao unknowingly helped in this by disposing other "heroes".
He is so weak because all the churches were abandoned, people only treated the story as a folk tale to cash in on.
Even if they visited places or bought merchandise, it was for their personal enjoyment.
Thought Big Sis was a fairy, but she said "human", hmm... yeah and she can't cook anything except charcoal ; )
Seems like there's no option, if the Demon Lord wins or they form a truce and never meet each other, demons will let loose and it's the age of them ruling over humans with those that are perverted sentient, consuming other lower demons.