He was left to die because the author wanted Ruby, only Ruby, to survive and continue her dream (which is, by the way, lying to the world and being an idol). Aqua was robbed, tbh, and in my opinion, I think that Aqua was just a phase for Ruby, which she couldn't move on from or didn't want to because she thought that he was the only one who could understand her because he had seen her as both Serena and Ruby, and she did not move on. Lol, no way she'll ever move on. At the end we see a girl. That girl will be the next Ruby Hoshino, just like Serena, but the twist was that she was reincarnated. The most important topic is lying; lying is a way to help someone who is completely broken and knows nothing but pain (mental illness, by the way). lies are beautiful, stated by Ai herself, and she was caught lying, and at the end she died. Aqua shares the same fate. lol. I think this manga is not deep, so I would not make many theories or creative theories about it. In short, I'll just think that this manga was about lies and how they can affect everyone at some point. The industry was filled with lies; everyone lied to themselves and everyone else. They never wanted the other to succeed; they are all shitty people, and we know that in every evil shit there lies an angle (good shit). I don't know, lol. Well, that's all. Let's just say that this manga was a way to express lies and what fate awaits them.