I don't agree with that. While Ryu was hyped up, all that hype was for his fight with Yuta. And his backstory was also very simple and doesn't need elaboration, he reigned as one of the greatest of his time and found himself unsatisfied. We were given a very sleek but full description of the guy, the idea that he had more in his past is an odd one. And while I of course think he could've totally had more to add to the story, I also think he already added plenty as is. He helped give us a good idea on the strength of one of our main players in the culling arc, Yuta, and gave us a entertaining fight out of it. To try and keep him relevant after all that isn't really needed. He's met his goal for the culling games, he's not the most dynamic character, and he's a good benchmark for the audience. Really isn't much point in keeping him on with the culling game being revealed as a farce. Mundane deaths like this is a staple of JJK, and I think reflective of its themes.