Looking at the manga perhaps the case can be made that it started going downhill at about the time the fights with yami got relevant. The reason being that kenichi himself got irrelevant to the grand scheme of the plot. Specially after the kano sho arc, after that one master fights got specially relevant. Before that the manga was about kenichi overcoming bullying and progressing through martial arts, it was about his own struggle. But then with the advent of yami things changed in the manga. Master fights were awesome however the issue is that kenichi did not have a role to play in them. Kenichi's fights early in the manga were important because they contributed to the grand scheme of the plot which was actually about kenichi. Then this happened.
From that point onward the manga stopped being about kenichi and overcoming his own limitations. Of course, that was a factor in the manga BUT it was no longer what the manga was about. From that point onward kenichi started fighting but in an entirely different context from before. Kenichi was taking stand against a massive evil organization that aimed for destruction. His masters took the stage and they fought against them with all they had. However as the manga advanced it was clear that the whole thing was a vanity project for both sides. Yami didn't have to fight ryozampaku, they did so because of the katsujinken, because of some principle. As I said, a vanity project. Kenichi's did have a bit of the overcoming his limitations but ultimately their importance was in said vanity project for both sides. The ryozampaku guys genuinely thought they were fighting against yami however they were ultimately a group of 6 people who through 99% did not even slightly dent yami. Alexander, diego, and multiple yami were defeated but not a single defeat even slightly caused damage to yami. And kenichi fought the disciples but that ultimately meant nothing because it contributed nothing to the grand plot of fighting yami 99% of the time. I mean, kenichi beat boris, ethan, kokin, radit and several others but that never actually hindered yami's plans. In fact, it was conceptually impossible for it to actually hinder yami in any way. Even the events from when he fought kokin were not all that relevant. At best it mildly annoyed yami. And it was nijima who actually did all the work, kenichi was fighting a battle that did not necessarily have to be fought.
And then we get to the end of the manga. The forces of good and evil join in a massive all out war when every man and woman counts. Kenichi gets to be important to the grand scheme of things because he is the only one that can stop kajima from killing nijima. Kenichi wins but the missile is fired anyways which beats the purpose of the entire and dumps the corpse in a septic tank. And the elder comes out miraculously and saves the day. Kenichi's only real accomplishment gets vastly overshadowed by irrelevance and the sheer absurdity the elder represents. The main issue with the manga is that it turned its plot and main character for close to 300 chapters into a weak subplot and a vanity project. It killed the actual importance of the main character. And it ends with kenichi supposedly becoming a master in the future, with kenichi finally attaining relevance... and we never get to see it. This was a great story but 300 chapters ago it completely deviated from its plot of a young boy struggling to overcome bullying and finding his inner strength and it turned into a manga about fighting a massive evil organization. However by the end of it it was neither.
From that point onward the manga stopped being about kenichi and overcoming his own limitations. Of course, that was a factor in the manga BUT it was no longer what the manga was about. From that point onward kenichi started fighting but in an entirely different context from before. Kenichi was taking stand against a massive evil organization that aimed for destruction. His masters took the stage and they fought against them with all they had. However as the manga advanced it was clear that the whole thing was a vanity project for both sides. Yami didn't have to fight ryozampaku, they did so because of the katsujinken, because of some principle. As I said, a vanity project. Kenichi's did have a bit of the overcoming his limitations but ultimately their importance was in said vanity project for both sides. The ryozampaku guys genuinely thought they were fighting against yami however they were ultimately a group of 6 people who through 99% did not even slightly dent yami. Alexander, diego, and multiple yami were defeated but not a single defeat even slightly caused damage to yami. And kenichi fought the disciples but that ultimately meant nothing because it contributed nothing to the grand plot of fighting yami 99% of the time. I mean, kenichi beat boris, ethan, kokin, radit and several others but that never actually hindered yami's plans. In fact, it was conceptually impossible for it to actually hinder yami in any way. Even the events from when he fought kokin were not all that relevant. At best it mildly annoyed yami. And it was nijima who actually did all the work, kenichi was fighting a battle that did not necessarily have to be fought.
And then we get to the end of the manga. The forces of good and evil join in a massive all out war when every man and woman counts. Kenichi gets to be important to the grand scheme of things because he is the only one that can stop kajima from killing nijima. Kenichi wins but the missile is fired anyways which beats the purpose of the entire and dumps the corpse in a septic tank. And the elder comes out miraculously and saves the day. Kenichi's only real accomplishment gets vastly overshadowed by irrelevance and the sheer absurdity the elder represents. The main issue with the manga is that it turned its plot and main character for close to 300 chapters into a weak subplot and a vanity project. It killed the actual importance of the main character. And it ends with kenichi supposedly becoming a master in the future, with kenichi finally attaining relevance... and we never get to see it. This was a great story but 300 chapters ago it completely deviated from its plot of a young boy struggling to overcome bullying and finding his inner strength and it turned into a manga about fighting a massive evil organization. However by the end of it it was neither.


