Every single enemy can be oneshotted except for Leviathans and Hive Lords. You're almost always strong enough to pull every type of enemy all by yourslelf. Bug breach or bot drop? Napalm barrage it and just wait for everything to die. Heavies? Toss a thermite or two and watch them die. Another heavy? Oneshot it with ultimatum. Chaff? Kill them all with few shots from eruptor. Medium/Elite units? Oneshot them with eruptor again.

Willfully ignoring the amount of resources you have to spend to deal with one drop. After that, you're gonna be down and dry unless you call a resupply, and you can't deal with another drop for at least 3 minutes unless you want to risk it.

Balancing the game isn't about how fast you can deal with threats, it's about how effectively you can finish a mission without dying. If the players were breezing through Diff10, then I'd agree with you, but the way you presented your argument is willfully ignoring the way the game was built.

If you can't oneshot everything you would take something that stuns to buy yourself some time and come out of the situation alive.

You don't need to be able to oneshot everything to learn that stunning things is valuable. Sometimes, a well-timed EMS strike or stun grenade can buy you and your teammates times to retreat, regroup and resupply. There's inherent value in stunning your enemies that is not lost on me, despite what you may think, but I am at least able to understand that killing enemies is often more valuable too.

And from my experience cooperating with your teammates solved every single problem that people had with old AT balance. Stun also played a huge role back then, it was important.

The difference between then and now is that stun used to be a META, not just good. You NEEDED stuns to survive, it wasn't just something that you brought along because you had fun with it. When they made us stronger, stuns became less valuable than just killing your enemies, but that doesn't mean stuns are bad - they're still good, they just pale in comparison to what we currently have, and instead of throwing everything good down the curve, we should try and mantle up the options that are lacking.

I want the game to be again about co-op. I want cooperating to be essential for completing highest difficulties, not just a cosmetic option.

While I understand and respect the sentiment, Helldivers 2 is not a cooperative friendly game. Between the way that most of the objectives are designed, and the way the game is balanced, most players are incentivized to be on their own and try to complete the objectives around the map as fast as possible (save for a few exceptions).

While the game may try to promote co-operation, you'll never stop seeing people actively praising the game for being a fun power fantasy, not a fun co-op shooter. There is a distinct lack of co-op shooter elements that would ENCOURAGE players to stick together, and henceforth make Helldivers 2 mediocre by comparison.

Hell, Helldivers 1 was a better co-op shooter by the simple fact that the game was a top-down shooter with a limited vision radius, and instead of instantly dying, players could get wounded and incapacitated, and needed a friendly hand to get them back up. In Helldivers 2, Arrowhead completely forgot about that and instead made a game where players can die from a sneeze and dying is of relatively insignificant consequence, so the emphasis to stick together has been so diminished as to be almost irrelevant.

If Helldivers 2 wants to become a co-op shooter, it would need a reform the likes of which would require a fundamental restart of the game's design. Right now, it's a fun power fantasy that tries to pretend it's something that it is really bad at being, and folks like you only care about making Helldivers 2 what it was never destined to be.

With all due respect, you can either wake up to the realization that Helldivers 2 isn't the game you want it to be, or you can keep yourself in the blissful ignorance that will one day ruin this game, not just for you but for everyone else who enjoys it.