Oh, if this is a success like you claim then the author will drag it as much as possible and till the interest drop, only at that point the plot will move toward the end.
On a side note, is odd that this manga have reprints, i mean, these are such stories that you read once week after week till the end, and after the end you wouldn't never back to read it again because the resolution of the mistery is the whole point, there is not real development before, you read the first chapters and the last chapters and you have the whole story while you can discard everything in the middle.