It is established in the story that Miyako negotiated her exit from the group, which means that the management had plenty of time to promote someone from their trainee program or come up with a good story on day 1 of how the group would go forward with just the four members. They could have done some press availability with Miyako where she give a happy story about how she wanted to focus on her new career in real estate management, and then give her a proper graduation concert. This would have avoided all the problems they're trying to deal with now.
I reread chapter 9 recently since it's in print and it seems like the exit she chose was more of an "escape plan" vs the typical graduation from a group like you mapped out and there's reasons for that. I don't think she had it in her to fake it anymore. Not a moment more even if it'd have served herself.
She had it drilled into her that her real self couldn't be seen. Down to policing emotions and mannerisms. Once she reached her breaking point on that her ability to assume the fake persona again for media would have felt impossible. It was the key conflict.
If I remember right her first/last/only public word on the subject was said on stage and then she was gone. She didn't let herself be further scripted more than the absolute minimum to get out even if the consequence was the non-contact rule and contributed to the rumors. She's aware of what's being gossiped about and feels guilty about some of it unfairly reflecting bad on the other group members, but if she was in a good mental state a whole lot of things would have been done differently or not necessary at all. She didn't hate her coworkers, the fans, or even the work. It was what the director did to her.
In a way it feels more like getting out of a cult. She was under the thumb of a controlling adult as a teenager, under the eye of the public and living in a restrained way beyond what the others dealt with, and eventually she got herself out. But she still is living by many of the rules of a confined world. She's come a long way, but the director just has to appear and she starts to slip back into the old mindset. By the time she got back home she was like a kid again and the world was ending over something that shouldn't feel that big.
Of course it seems silly to people who didn't grow up like that, but to her this is what she knows. At least she has someone from the outside with her now who will have a different perspective. Someone who has been with her long enough to understand her lived in anxieties too. And while we haven't seen them talk in detail about it yet her already thinking about the car purchase and the importance of making big decisions together makes me think whatever they do will be based on what they decide together. Both sides of the story are coming together which feels nice to me.
