It isn't finished because it was never meant to be finished.
The whole game is meant to be an infinite loop meant to trap Blick Winkel in Kokoro/fake-Satoru's bodies. So thatheyou can butt out of other people's lives and more specifically, stop causing real-Satoru's sister to have DID
At least that's my take on the game.
Yeah I reach similar conclusion. I know how Nakazawa feel that it's unreasonable to expect from the readers to do that much to reach this conclusion.
I think he meant it's unfinished in this sense, plus the fact that some info was hiding really deep inside the vn. But I think it paid off very well in the end.
GA also found some interesting info that support this way of thinking in different side materials: # & #