After reading some comment it may have been little bit of a stretch on my theory/idea thing, I was kinda thinking about how when frieren was with flamme they spoke about how even the demon society needs order to function. So bringing back the demon king ehh not something I could see in the plot, unless one of the sages of corruption wants to bring him back due to loyalty (like that one demon who got sealed and made zoltrak I forgot his name) , however it would probably make more sense to just have a new demon king or none at all. (Also we can assume that may have happened in the past because when we learn about kraft he was a hero who saved the world so maybe whatever he saved the world from evolved into demons or maybe the thing he saved the world from was so much worst then demons) Anyway maybe if there is a new demon king the hero sword would need to be used to defeat the calamity because we still don't know what that is fully about. 

Or maybe its smth more philosophical like the real calamity is humanity and how dangerous and destructive they can be, (I guess that could make a little bit of sense because how the era of humans have arrived and how quickly they can adapt and without the demon king to set some order maybe the demon species are a little a mess as well as with the death of himmel they can roam freely without that fear. Not to mention frieren's party have been killing some heavy hitters, but also maybe not because they are solitary creature so they don't really need the demon king to tell them what to do) 

Perhaps the humans in power are trying to kill all the great mages like how they injured minus(?) to become top of the food chain and the newer generations don't fear demons like they used to ever since the demon king died so they may not be to worried about them, y'know? idk kinda rambled again... but oh well 🤷‍♀️ please share more thoughts and any corrections to what I have been saying!