Muzan is the guy that can make more demons, but i think UM1 is much more fearsome than him. Idk, im pretty hyped with this fight and if Muzan dies in this arc ill be just "eh, main objective done, neeext". I hope the manga won't be too long, but as others mentioned, another arc focused on Nezuko and the fact she can walk under sunlight would be a good way to end it.
Also Genya my boy, you gonna have problems :v or be the next arc problems XD either way, its a very good manga so far!