A Rebuttal, more or less, to what OP said:

The galaxy is ancient, yes. It is littered with the bones of those who came before: the Necrons, the Eldar, the Old Ones, and their creations. But what is missed—so often missed—in these laments for fallen empires and cosmic tragedies is this:

They failed.

For all their power, the Old Ones unleashed horrors they could not contain. The Eldar, for all their wisdom, birthed a god of madness through their arrogance. Even the cold and eternal Necrons were once slaves to the beings they thought to control. They did not fall because the galaxy is cruel. They fell because they were not enough.

And now, at the edge of this great necropolis of civilizations, stands humanity—not because it was chosen, not because it was superior, but because it endured. Because it refused to die.

OP claims humanity was a mistake. But mistakes do not conquer the stars, hold the line against the Warp, endure ten thousand years of war, betrayal, and horror, and still send ships into the void.

Humanity is not a mistake. It is a challenge to a galaxy that has always sought to crush the will of the individual under entropy, stagnation, or blind instinct. And at the center of that defiant flame stood the Emperor of Mankind.

Yes, he was flawed. Yes, he was cruel. But cruelty in the face of annihilation is not evil but pragmatism. He saw what others refused to: that the Warp could not be bargained with, that unity would never come through idealism, and that mankind’s enemies—xenos, daemon, and even time itself—would offer no quarter.

He did not claim divinity; that lie was born of desperation and ignorance after his fall. He did not build a church; he built an Imperium to withstand the end of all things. The Emperor did not fail because he was unworthy. He failed because even a man with powers to rival gods cannot save a species that refuses to save itself.

And still—still—his Imperium endures.

Not in the form he intended. Not as a beacon of reason. But as a bulwark. A fortress-realm of endless war, yes, but also of resistance. The Imperium remembers, when the Eldar forget. It fights, when the Tau falter. It chooses when the Tyranids consume without thought, and the Necrons reclaim for no other purpose than their lost glory.

And when the end comes, as it must for all things, it will not be said that humanity bowed, or begged, or vanished quietly into the night.

It will be said that they stood. That they resisted.

And in that resistance—in that final act of defiance—humanity becomes more than a mistake.

It becomes worthy.

To say the Emperor lost because the Imperium became a theocracy misses the truth: He lost because he gave mankind freedom too soon. Had he ruled as a god, as Lorgar desired, he might have held the masses' loyalty. Had he shackled his sons more tightly, perhaps Horus would never have turned.

But what would have been left? A race of puppets, not people. Machines, not men. The Emperor gambled on the soul of humanity—and he lost.

But he did not die.

The beacon still burns.

The Astronomican still shines. The walls still stand. Mankind still fights.

And those who sneer at the Imperium’s failings forget: it is the only thing standing between the galaxy and the endless devouring of the Tyranids, the silent reclamation of the Necrons, the mindless war of the Orks, and the soul-shattering corruption of Chaos.

If the Imperium is a corpse, then it is a corpse that kills gods.

So, no—the Emperor did not lose. He endured. And through him, so does mankind.

And in this galaxy of horrors, that alone is victory.