What I find interesting is that Hellele has use for Goddess essence. So we know that both demons and heroes used to be much, much stronger. If Goddess essence is what gives heroes power, does Hellele's essence make demons stronger? What if the Goddess also has an interest in Hellele's essence? What if killing the demon lords and other powerful demons does something with their essence?
Another interesting fact is that the Goddess's "blessing" is concentrated into only one or two individuals every generation: the hero and his priestess. What makes the hero special is that his blessing is hereditary. If masturbating reduces the blessing's potency, what do you think happens when they have actual not-"ruined" orgasms? Maybe the reason why the demon king always dies is because the act of siring an heir weakens him. Alternatively, what if it's a matter of quality over quantity? Maybe demonkind collectively have the power to rival the hero, but their individual quality is low and their inability to make good use of it (or unwillingness to die for their race/king/nation) makes it easier for the hero to win, since he doesn't expect to die. There's no precedent.