Also, as much as I understand the complaints about Quiet disappearing forever, they kind of feel like they're missing the point. Quiet is gone, she's dead. Of course that sucks; that's what is so tough about it. It comes out of nowhere (unless you spoiled yourself ahead of time) and hits you like a ton of bricks.
And there are repercussions for her death. In fact, it's the only real repercussion that really seems to stick on a narrative and gameplay level - the game makes you realize the loss through getting used to her usefulness and taking it away. It's arguably the only effective character beat of the game in my opinion.
I don't want to say it's exactly analogous (and I'm hesitant to make this analogy because it's probably going to throw the whole discussion off-track), but it's almost like complaining about Aeris dying back in 1997 and how you put so much time into leveling her up. Yes, that's a gutpunch. But it isn't an inherently bad design decision to prevent her from returning - it's the gameplay complementing the narrative for once.
Where are you getting the idea that she dies?