Long post ahead. tl;dr - This ending is very effective at framing the whole series as a criticism of the Japanese entertainment industry, as well as the futility of the criticism itself.
After letting this ending stew in my brain for a little bit, I have actually developed an appreciation for it. Let me explain...
From the beginning, OnK has been praised for its frank portrayal of the Japanese entertainment industry including all the backroom dealings, the power of connections, and the profit motives behind stoking the fires of drama. If we zoom out on this story as a whole, we have basically ended the story in an almost identical place to where we started; a young star idol without any family members has become an object of fascination for the public. To keep this up, the idol lies, both to the public and to herself, viewing that as the way to live up to the expectations of others.
Almost the whole story that happens between those two points happens within the bounds of the industry itself; a movie here, a play there, a music video, a concert, etc. All the drama that Ruby and Aqua (and everybody else) dealt with in their lives ultimately ended up being monetized by the industry machinery: that dating show got really popular after Akane's trauma, Aqua redeemed Sweet Today by channeling the stalker that killed Ai, the life and death of Ai depicted in the movie (the director even won an award!), etc. It's telling that after all of that apparent success within the industry, everybody left standing seems incredibly sad this chapter.
So, I think an interesting way to view this series is that it depicts the wheels of the entertainment industry slowly turning, grinding people underneath it, to provide the dramatic fuel for the industry itself. It is some kind of perpetual machine that takes in human tragedy and leaves further tragedy in its wake in the pursuit of profit.
The ultimate irony from this interpretation is, of course, that Oshi no Ko itself is being used in the same way. It can criticize the entertainment industry all it wants, but it isn't going to bring about any lasting changes because it is an incredibly popular (and profitable) franchise. It reminds me of the famous quote by Audre Lord, recontextualized in this instance for the entertainment industry:
For the master’s tools will never dismantle the master’s house. They may allow us to temporarily beat him at his own game, but they will never enable us to bring about genuine change.
In addition to the manga, there is anime, merch, novels, stage plays, live action adaptation, etc. All of these ventures are making money for the people involved. At the same time, it has dredged up tragic victims of the past as well as created new ones. All of this makes the industry criticisms in Oshi no Ko ring just a bit hollow.
