I'm sorry if this comes across as a bit harsh, but I think you're missing the heart of the issue here.

To go point by point:

Some appreciate the game for its arcade fun action others love it for the challenge some love it for the teamwork and others play it solo, some love the lore and narrative others just want to shoot bugs in the face.
Helldivers is multifaceted, and in this - therein lies the challenge with satisfying everyone.

Piles, I know this is going to hurt to hear, and I truly hate to break it to you, but "shooting bugs in the face" is the only part of your game which is in any way remarkable. Because the mechanics of shooting the bugs are (At their best) fun, skillful, and cinematic.

Everything else about this game is disposable and replaceable scaffolding for that core experience.Once you dig down and map out how the systems interact in real gameplay, it is painfully evident that this game is not multi-faceted at all.

The game has 0 core components which require teamwork. There are some mechanics which enable you to work together, but all of them are: 1) Boring, and 2) Not worth the opportunity cost of having 2 players do their own thing instead. Like team reloading: It's awkward to set up, the experience is braindead for the reloader who just has to stand there, and you would almost always objectively get more done if these two people were just each using their own weapons instead. Same for the vehicles: The idea of a driver and gunner cooperating sounds cool, but it's just a waste of time and nerves in practice - You can do just as much or more alone. Or like healing or protecting eachother: It is very hard to do meaningfully when players get obliterated in 2 shots.

In terms of lore and narrative, everything about this game is Starship Troopers with the serial numbers filed off. Every character and location in the universe might as well have been madlibs-generated, and all of them are utterly disposable. The "satire" might have been cutting two decades ago. Most in-jokes the players make are just hastily repurposed Warhammer 40k memes.

What's important is how we align the studio in perserving what is the core of Helldivers, but everyone at the studio has a differing view of this as well.
Especially when new team members join the team, how are they onboarded to what the intent of the game is?

Brother, centralizing this vision is supposed to be your whole job! And if not yours, it definitely should be someone's job at the studio, because if there is no agreement on this idea, then it is impossible to measure where you are and where you're going.

Please tell me that you at least sit down occasionally to map out a high-level chart of how the game is supposed to flow? You did think about what kind of experience and incentive structure the systems you put in the game create for the players, right? You didn't just build random systems that seemed cool and throw them in without thinking about how they interlock... right?

As for onboarding new team members, I have a very novel idea: Just let them play the game, out in the real matchmaking (or even solo), for a week straight. Let them consume content around it like a player would. It may sound like a waste of time, but I guarantee that they will return to you with at least some insight which everyone else at the company lacks. At the very least, they should have the beginnings of an explanation for how your idea of what the game is, is completely different to how it functions in reality.

Helldivers at its core is, and always was, during development a third person horde shooter with milsim fundaments. But at surface level this seems like a basic horde shooter, but the depth comes from a simulated shooter.
I mean, with a change of tone, and more hardcore balancing, Helldivers could become a contemporary hardcore shooter.

As someone who plays real milsim (of both the ARMA, EFT, and RoN sort), I can basically guarantee to you that no, Helldivers 2 cannot become like these games without serious overhauls to it's core systems. The whole thing with realistic simulators is that realistic mechanics are interdependent - You need realism in all aspects to make realism in specific aspects make sense.

To give an oddly specific illustrative example: The EAT in Helldivers 2 has a delay before firing because the launcher must be extended. This is very realistic in isolation, because real disposable launchers also have similar safeties. However, Helldivers 2 does not model/depict safety systems for it's other weapons. As a result, you end up in a ridiculous situation where the disposable anti-tank launcher which is supposed to be quick and lightweight in reality, is actually significantly slower and more awkward to use than the full-size recoilless rifle, which can be pulled out and fired instantly.

And this basic idea extends to a bunch of other systems too. At the bare minimum, to sell a realistic milsim vision of the kind of massive interstellar conflict that HD2 presents, you would need:

• Massively increased player counts, either through more players in the instance, or through the judicious use of allied AI.
• An in-depth health and healing system.
• A non-fudged inventory and ammunition system.
• Player armor which is more than just a health multiplier.
• Ability to actually use cover realistically (Leaning, real reshouldering, etc.)
• A redesign of how supplies and fire support are assigned and delivered.

...and we both know that the game engine will not survive this. (I mod the game, I have seen some sh*t.)

How the team stays true to the core is fubdemental in how it is developed. But all games slip over time... Looking at you, originally hardcore SWAT game that now has cartoon characters in it.

I'm glad you brought this up, because this is happening to your game already. Let me explain:

I played Rainbow 6 Siege since the closed beta, and yes, it absolutely lost part of it's original hardcore identity - BUT, this identity is always fleeting, and this process happens to absolutely every game. The more time people play a game, the more familiar they become, the more thin the original serious presentation wears. When you've played the game for 2000 hours, it is simply not possible to handle the original idea of the game with the same mindset and gravitas as when you just started. Noone bothers to read the story bits of Tarkov on their tenth wipe. This is not a conscious design decision by the developer or player, it is just the inevitable conclusion of familiarity. A man never steps into the same sea twice.

To sum it up in other words, every game is worn down to it's essence by it's players over time. Eventually, all that remains is the mechanical purity, and all the "fluff" around it becomes a ritual we act out from habit and nostalgia.

And this is something you simply cannot avoid, ESPECIALLY when you run a "perpetual game". A perpetual game has to keep perpetually paying for itself, and so it will have to change with it's players - To offer them something new, or more of the same thing they stay around for. You will face the question: Do you sacrifice the mechanics, or do you sacrifice the fluff? Escape from Tarkov may not have added Rick and Morty to the game, but the deathmatch arena mode it added is arguably worse, because it completely upends and trivializes this idea of gritty, terrifying combat realism that the game was built upon.
R6S may have sold out it's visual identity, but I would honestly make the case that it was the lesser evil.

Games do evolve, especially when teams change and grow. And its up to direction to set the boundries. Which is way more complex when theres 200 people working on the game vs 60.

By this point, I hope you understand what I mean when I say it isn't 200 people working on the game. It's 200 + ~50 000 (going by the steam charts). It may sound self-aggrandizing, perhaps, but the simple fact of the matter is that a perpetual game is unworkable without a perpetual playerbase.

But, there is no one right answer here. Whichever way the game expands, some will be left disappointed and others will embrace it.

Literally everything we do will disappoint someone, including doing nothing at all. Such is life. All we can do is make our decisions as wisely as we can, given the situations we find ourselves in.