No magical macguffin ending so its nice Aka didn't pick easy option. I think this ending did well because it goes back to being what an idol means. To shine and gives hope to its fans, even with love called lies. That said the resolution feels a bit rushed since it just several wordless pages showing other characters conclusion.
Anyway if OnK ED1 tells the foreshadow Aqua's conclusion with him drowning, then OnK ED2 foreshadow Ruby's ending with her running and moving forward despite her sadness.
At the end of day Sarina/Ruby finally achieved her dream of being star idol like Ai. On the grim reality side its Aqua's death causing more sensation to Ruby to boost her career, and on the light dreamful side its Ruby's perseverance to keep going on despite the tragedy that inspires people to love and admire her. And she did by copying her mom ability to tell lies to herself.
All in all its Ruby basically succeeding where Ai fails at.
Lets think about.
What Gorou's regret? His regret unable save Sarina and let her becomes an idol.
What Sarina and Ruby's dream? To become an idol like Ai.
What Aqua's desire in the first volume? To avenge Ai and kill her murderer, Kamiki.
All that achieved one way or another. So in a sense Aka didn't lie when he said the ending is exactly what he envisioned from the beginning.
The reason why Ai falls is because Kamiki killed her. His obsession is the biggest and final obstacle to this series. Ai, the manager, Miyako, Ruby and Aqua's dream falls is because of Kamiki. The moment Aqua removed him, their dreams becomes true through Ruby, even if takes Aqua to sacrifice himself.
Now you ask does Aqua have to die? Well think about it:
How can Aqua achieve what he set to do if he survives? Ask Akane's help to create a perfect crime? Ignoring that also dirties Akane's hand in murder and ignoring the impact on her psychology onward, its also assuming that Akane can truly create a perfect crime, something that Kamiki who is just as intelligent proven to be false as his perfect crime on Ai got busted years later.
Hell I'd argue we should've seen the ending coming the very first moment Aqua said he wants to kill Ai's murderer, cause how he is gonna do so without dragging Ruby's future as an idol? I imagine this was his plan from the get go or at least after he convinced that Ruby is really serious becoming an idol.
So Aqua chooses to commit double suicide with Kamiki, but the different with Akane's attempt at suicide is that Akane did it out of despair and her doing so saves no one, while Aqua did it to save Ruby, in other words Aqua martyred himself.
Honestly my biggest gripe is that Ruby and others went from despair to having hope again in span of two chapters, and it was depicted wordless with just pictures. Its not particularly satisfying and usually Oshi no Ko will have introspective dialogue of how modern society works. The last chapters missing all that so it just feels rushed.
Also in the end the crow girl are just plot device for the reincarnation stuff and there's no explanation about them. Though someone mentioned about special chapter so if its true maybe it'll be there it gets revealed.
All in all not too bad, its a sad ending but its ties the theme well so I accept it.