Flight attendant, huh. I think it would be most likely she went out on a plane one day and died there, and Kindaichi kept it from his friends, or they know about it but try to say good things to cover it up. Even when Reika asks Kindaichi about her, he doesn’t give a clear answer as to how Miyuki is. How would she know what and where Kindaichi is doing and staying at. It makes sense from the very first chapter as to why he was depressed and didn’t pursue the path of detective. He can’t do it without her presence. I think the texting is a figment of his imagination. He misses her so much that his thoughts are presented with text. It’s very obvious to me that something happened to her, and I would not be surprised if it’s found that she had died long ago.
Even in the original series Hajime became depressed when things happened to Miyuki. However, he didn’t give up bc she was there alive and well to cheer him up. Knowing that, I can assume something devastating happened that made him not like solving cases anymore. Like idk, it could have been a bomb on a plane in a case he was involved in.
But I did wish he stopped saying “Unfortunately, all cases have been solved” haha just solve them already xD