i read shibito no koe wo kiku ga yoi back when it was first being published here and i just got done with my rereading and i honestly don't really get why people are so frustrated with the ending. surely, uguisu probably had to quickly wrap it up after being told this would be the last volume so it was a bit rushed in some aspects, but other than that the ending was pretty fitting to the series.
jun let go of his guilt and regret over hayakawa's death and move on with his grief, and that was the focal point of the entire manga. sure, it was an episodic horror anthology, but the narrative thread woven throughout the chapters wasn't about psychic powers or ancient gods, it was about the relationship between jun and hayakawa.
it can be a bit frustrating that we will never truly know what happened after they entered the fog, the death snakes disappeared but it may just be that they were done feeding on their auras as they would die just then. jun got to develop his powers by leaps and bounds through all the troubles he went through and the connections he made were also proven to be useful in his attempt at avoiding an apocalypse as much as he could. even so, ghost was pretty much done with her preparations and the sea god's awakening might be inevitable at this point.
also, that might be a personal bias talking, but this isn't a battle shounen series. seeing jun fight the sea god or something of the sorts would be very tonally dissonant. i don't think there really was a way to go beyond what we were shown without falling flat