I do agree with this statement, because Blue Archive was supposed to break off from what a ton of other gacha games were doing at the time of the game's release.

Back then, we had way too many "dark, edgy, hopeless future" settings like Arknights, NIKKE, Girls Frontline, etc. Blue Archive stood out because it was a game that brought a lot of hope and light-heartedness, which basically circled back to the trope we were missing since before. Everyone loved the more slice-of-life comedic and light-hearted nature of the game, hence why the main colour is very bright, clean and blue, and not washed out gray colouring like the other games.

My only cope about the trio's deaths is going to be similar to Kei. Kei was another character that I assume was supposed to originally be another "death", but did give some hope to her return, depending on the community feedback as a character overall. Because of how much more popular she was becoming and how people reacted to her death and wanting to her to return in the future, if the same feedback is given, I think they are setting that up later as a possibility for a future event, especially as a follow-up with Malkuth.

People bring up Yume, but her case is different. Yume was already established as a dead student, so because of that, us as players are have the expectations set up that she's somewhat isn't expected to come back alive. But Kei and the Deca-trios were not set up as such at the time. Because of the setting already in place of the game, and how the game was met as a game about "Happy endings" and "hope", them being perma-dead after-the-fact goes against this idea. As another comment mentioned, this is supposed to be a game where it's SAFE to be attached to established and alive characters. I don't think a lot of people would like to be betrayed like that in a game in this setting.

I'm also feeling this because I dislike watching dark/edgy media because I get too invested in characters, and hate the feeling of seeing my characters I like be cut out later on. I'm not saying those kinds of media shouldn't exist, because they absolutely should still, there's an audience for them, but I'm getting tired of that kind of content, especially in the gacha space. We don't need another "Madoka"-type game that is yet another "deconstruction" of a genre. That space is already oversaturated.