to understand the depths of darkness one must first have a concept of daylight.
my number one frustration with the grimdark of the 41st millenium is the steady corruption of "it's an awful and authoritarian regime" from "because we're modelling this after judge dredd & star wars's empire" to "because things are genuinely so bad that unrelenting and horrific fascism is the only way to save humanity in this time of crisis,"
because the former is the fanfic of a bunch of nerds writing about their favorite media properties in the late 80s, and the latter is literally just the language that fascists really use in real life nowadays but broadened "out to all of humanity*."
like i've said before - think that the interesting answer is that the imperium is smaller than it purports to be, and it is as cruel as it tells on itself to be, but in service to maintaining the power structures that want to cling on for dear life. not because the galaxy is ready to destroy humanity and absolutely not because of any "necessity" of this cruelty.
I THINK THAT THE CAPACITY FOR ACTUAL GOOD AND TRUE HEROICS MAKES ALL OF THE EMPIRES - NOT JUST THE IMPERIUM OF MAN, BUT THE AELDARI & THE DRUKHARI AND THE WORST IMPULSES OF THE TAU - LOOK WORSE BY COMPARISON.
I AM ALSO, ADMITTEDLY, A DEEP LOVER OF THINGS LIKE GUILTY GEAR AND FINAL FANTASY 14. I LOVE IT WHEN YOU REALLY CAN OVERCOME IMPOSSIBLE ODDS. I LOVE IT WHEN THERE'S A CHANCE THAT IT WORKS OUT, BUT THAT CHANCE HAS TO BE FOUGHT FOR WITH TOOTH AND CLAW AND WILL EXTRACT A DIRE TOLL FROM YOU.
ultimately, i guess, we're not so much "shackled" to it, as much as we're happy to use the grimdark as a counterbalance to our own desires for anime and video game heroics.