I share the same opinon as you, Erin-chan. Of course I find it unfortunate that this manga will come to its end soon (well soon... perhaps in a year or two, so I think that might be enough to answer (hopefully) all the questions) however, at the same time I am glad. Mangas with these typs of stories (mystery and horror most of the time) shouldn't be long running mangas with over 300+ chapters. The reason behind that is mostly because if it drags out too long, it will loses it touch, its view of the story itself what it was made for, adding perhaps unnecessary mysteries or goals that shouldn't be there in the first place or better said; it might lose its whole purpose and most imporantly, people will become uninterested in it also. Mostly these mangas have particularly one main goal and for that they only have to answer the why//when/how events and what they can do to find a solution or answer, kind of depends what the story is about of course. It kind of reminds me of Pandora hearts that has the same kind of genre (mystery and dark fantasy) for example. That manga only had 105 chapters (but then again it was a monthly manga), however that one ended on a high note imo. There was also a manga with a similar genre (forgot its name) that was dragged out too long and because of that most people lost its interested including myself, because of unnecessary building ups etc..
So yeah, I am quite glad with this announcement and hopefully it will end on a high note.