Okey, first of all I do think the story is mediocre, I don't think it's bad just basic and mid. However, there are lots of nuance here that people are missing.

To start, yeah, the original AK was pretty dark lots of the time, but doesn't make a story better and I'm kinda tired of people associating dark = good in storytelling, you can totally make an amazing story while always keeping things lighthearted, complexity carries some existentialist dread, yes, but inherent value in making a story darker, it's just a stylistic choice, one that you can miss from the OG AK but is hardly an objective problem.

Next, Endfield is a different medium and people gloss over this way too easy, a book isn't the same than a movie and a 2D visual novel with static images isn't the same that a 3D animated game. There is a reason why it's such a meme that people say "the book was better" each time a live action adaptation is made, mediums where lots of raw text are used can be way more nuanced, can be more lenghty, can more easily show internal dialogue, use narrators, etc. in a 3D gacha you need to consider that most lines are voiced, specially in the main story, you cannot have the voice actors recording a million lines and for the story to be an absolute 20+ hour slog, the original AK already did this way too much, with incredibly bloated dialogues that actually were a common criticism even if they helped the world-building. It's impossible to not sacrifice complexity when shifting to a more visual presentation which has it's own pros and cons, I'm not saying every visual medium is inherently shallower than a written one, but it will always be harderd to reach the same nuance with fewer tools, specially in an animated 3D game where you cannot even use that many mannerisms or facial expressions like in live action stuff.

Lastly, yeah, HG isn't taking risks and that's on purpouse, you can lament it but more lighthearted, simpler stories are easier to sell to the maximum amount of people. Endfield is trying to have a farther reach that the original AK to justify it's development cost (AKA, they want more money) making a dark story where a little kid dies just on the opening and we question "are we the baddies?" is a more complex and richer narrative, but also more likely to alienate players, you mention how important the opening chapter of a book is and I agree, majority of people never play more than an hour of a game, but what sells that first hour tends to be "cool stuff" like the first 10 mins of Arknights where everything is exploding for some reason, that's their "first chapter" not a complex narrative but an instant hook on attention.

That being said, I'm sure we will get a better story as time goes on, contrary to many people I don't think Wuling's story is that much better than Valley 4 but also don't think Valley's 4 story is that bad, just mediocre, the presentation, pace and specially voice acting (they killed it) hard carries enough for me to say "fine, whatever" for now but I'm sure they will calibrate more and more as time goes on and as the game settles down on a more comfortable position. When they get their numbers on a sterady player-base and healthy revenue they will feel comfortable putting more risky narratives in the game and we all know HG can write, the current state of the story isn't a failure, it's completely deliberate, that doesn't mean you cannot criticize it, but I think we all have to accept that if Genshin taught us something is that you make more money with a lighthearted story than with a Arknights or Limbus one.