My understanding is that in the novel, after the tournament he accepts the demons' offer to become demon king, and the story is his journey to fulfill that goal.
The manga getting axed meant this is kind of an "alternate ending" situation.
So basically, because the manga was axed, the MC just goes all "forget the demons, I'm just gonna do that thing I wanted to do at the start" while the gods, who are OBVIOUSLY EVIL, are still continuing their evil plots.
Just a question for anyone who HAS read the LN, does the MC keep his role or does it switch to another MC?