Even chimps have wars. I know they have to build up credibility for Sansei Shounin's plan, but the idea that devolving into mindless animals is a blessing because no more war or bad thoughts is obviously dumb and wrong.
Oh friend, I think you misunderstood Sansei Shounin's plan. He didn't do this because he thought humans would be better off without mantra. As Miyo has observed and lamented from her journey with him and her time as a virtual god, the prosperity and suffering of humanity are all meaningless to him. His only concerns are with his dragon waifu and the other gods — that's the main goal of the presentation, to save the gods he cherishes so much. The fact that mankind would lose mantra is just a side effect.
The discussion of what mantra means for humanity has already been covered before. Sansei Shounin feels repulsed at the thought of Ozuno becoming like those supplicants who rely solely on the power of the kami, which is factually correct considering the story is literally him going around borrowing sacred tools from gods. Mantra is the power to grant wishes, and Ozuno learned from Mount Katsuragi that people know not the weight of their wishes, recognizing that even he is no exception. His mantra to save Hitokotonushi brought a calamity upon Zen's village. It's reminiscent of that argument in Harari's Sapiens — the people who started agriculture knew not of its results or the splendor it would bring. Does the prosperity of distant descendants you don't even know justify the misery of now? Perhaps this is why Ozuno can understand Ryouboushi's words. He knows too well the weight of mantra.
Ryouboushi likes the world without mantra, and that view is sensible considering his very own situation. The very engine of the struggle around him is mantra. The people in the court don't fight for basic necessities like food and shelter. No, they fight for wealth, status, honor, nations — all of which are mantra, as discussed in chapter 22. Mantra is not just the power to grant wishes. Mantra itself becomes new wishes, new concepts that were not there before. And they constantly feed into each other, becoming a massive and irresistible torrent. Treating mantra as fate or history is quite appropriate, not only because people always look back to correct history into a story with a direction that aligns with their mantra, but also because every human being born is swept up by the massive mantra of humanity — money, nations, ideology, philosophy. These ideas existed long before you and I were born, yet here we are, continuing to fight and yearn for them.
This chapter somewhat mirrors Miyo's journey as a virtual god in the last volume. It too is a journey through the history of mankind. The difference? One is from inside mantra, as a human — Ozuno's journey. And the other is the journey looking from outside — the kami's perspective. They both see the two sides of mantra. Miyo's words: "My flood... my war... so many lives lost. But then... more were born, more than ever before." And Ozuno's: "Those things that twinkle brightly in the sky... Are they the fires of war that rage madly... or something that baffles the mind and dazzles the eye." Both refused to stop because others were waiting for them. "That journey is not over. Our wishes do not lie behind us. That is why we move forward." Susano'o, the wise, recognizes this drive, calling it love. Though I don't fully grasp what he meant — the four hearts, three black and one white oni heart, are still a mystery. Well, we can only wait and read.