Looking at 40k through the lense of purpose 1, yes, The Imperium is the only way humanity can resist chaos, because playing against a xenocidal police state is the only way to make playing as mindcontrolling daemons or world eating bugs feel guilt free. Everything needs to be awful, forever, and the Imperium fits that.
I feel like your stated position and your argument for it contradict each other. If the aim were really being able to play guilt free against the Imperium, then the Imperium having an excuse for why it is the way it is would be counter-productive. Thus, your point and your argument are in disagreement.
Yes, the Imperium is an oppressive, genocidal, totalitarian police state, a mix of space!North Korea, space!ISIS and space!Hitler. But that doesn't mean it
has to be that way. In fact, the Imperium is just one thing that makes the dystopia even worse. Every faction does. The Imperium doesn't have to be that way, but it is, because
everyone is despicable villains, because grimdark.
Except the Tau, of course. For the Greater Good!
Honestly one of the things that always comes up in these types of discussion is people conflating "A galactic polity the size of the Imperium" with "The Imperium of Man as it exists".
You can make a credible argument that the sheer scale of the threats in the 41st Millennium require an interstellar polity in order to muster sufficient forces to oppose, and I would actively agree with those. When an Ork Waaagh begins, the resources of a single solar system are not going to be able to turn it back without horrific casualties that make the whole place too fragile to resist the next threat, and that in the best case.
I mean, one can also make the opposite case: An empire of such scale as the Imperium, and with such an absurdly inefficient bureaucracy as the Administratum, cannot possibly be responsive to the
myriad of
local threats that exist at
every single corner of said empire. After all, the Imperium isn't facing one big enemy. It is facing literally thousands of crises at once, thousands of different, uncoordinates enemies, spread out over a region such that it can years to get there - and with the Administratum, entire
generations to respond. Having dozens of human polities each responding to their particular crises and threats in the region would probably be better for fending off said threat.
If you face countless threats and raids at far away borders, you have to rely on decentralization, like the Frankish Empire establihing marches at its frontier with considerable autonomy. Of course, one could say that the Imperium already
is feudal, but in truth, it's a weird mix of feudalism and totalitarian rule by centralized bureaucracy, the worst of both worlds.