Well it depends on whether Gotoge sensei wants to continue the series past Muzan. If so then I can see the series running longer, but of course it is always that when a manga runs past its initial concept that even more over the top story elements have to be employed just to continue, Fans start revolting and saying the series has lost its magic, so it is always a "double edged sword" so to speak.
Now I have an idea in mind should the series contniue even if they kill Muzan. Maybe the blue spider lily that turned Muzan into a demon is not from the natural world and is in fact a "demon flower" and then the story revolves around demons that are stronger and more ancient than him, plus they also come from the "world" or dimension that grew the blue spider lily. And another thought; Maybe if Nezuko turns back into a human she will undergo Demons slayer training too.
That's my wish though. lol
Edited by Phantom_999, 14 December 2019 - 10:20 PM.