Why is the manga not over man, we had a conclusion already
Not necessarily. There weren't any real plot threads tied together with the end of the last volume. We still had the medium shenanigans, the deal made with Death, and the police investigation to figure out. If it ended there, it would have felt like an axe.
it feels like the story is drifting from the original purpose
I disagree, seeing as how he's not on track to bringing her back (and quite possibly in a corporeal form rather than some spirit) and she's working on a way to return to him. The conclusion I'll guess is to be the starting point, except they journey together not in spite of their shared desire for death, but their shared desire to live.
We can see he's already grown past his desire for suicide in this volume, for example. Only a matter of time I think.