@Blip If that was the case, Yushiro should have died as well. Muzan is the root of all demons. Even with the modifications they made to be detached to his control, they are still fundamentally linked to Muzan. It's just that he can't control them.
It might be possible that they cleaned up the remaining demons. It's already been a month since Muzan died but with how much manpower they lose to kill him, it's very unlikely they were able to successfully accomplish it.
There's also a possibility that Yushiro tried to convert a few people into demons and one of them gone rogue. This said demon became a major problem in the present but we have to wait on the last chapter for what actually happens in the end.
This is my speculations anyway. It might be entirely different to what I have in mind once the final chapter comes.