I mean, the only way was to let go. And she let go. There was no other way (although I appreciate the small reunion wiht her biological father, if only because he was possessing Hyougo). And... man, I know the whole "Hyougo tried to kill her" happened like, 5? or so years before the finale, but, man... like, it's obvious you were caring for her more than just a job (even like... liking her?), and... I suppose it's that the final note that the author wants to reminds us that, if Hyougo changed, Muzan can do as well.
The ending is kinda weak... but I kind of wonder if this is because that's how this specific story had to end. Hatsune cut ties with Muzan, she is technically not his daughter, anymore due to that... so, what's the point of following it like this, after all the baggage (she already learned about her parents' death, for starters, Muzan is not a figure as such in this). Maybe that's how the author wanted to turn a new page for the whole situation and make Hatsune go through more things, as well?
So, I went to peak a bit in Wikipedia. Apparently we are missing an advance for ni Azuni (the actual title for the sequel is "Not the Magician's Daughter", so, lol) which was already being serialized by the time the last volume was released... and... which... I mean. Probably dayment cooooould have wanted to work in the sequel, but it's 6 years after that point and the group has been MIA since 3 years ago (or so says MangaUpdates).
The few things I'm reading about the sequel makes me want to have and read more, lol.