People seem to forget that Otonashi was NOT supposed to be in the afterlife in the first place. He already fulfilled his life, he was only there to fulfill others, mostly Kanade, which brings back to the question of who is "God" or who is in control of things to bring Otonashi.
Ehhh for all we know, someone else will step up to take that role now that he's back in the living....just as planned.