Since some still don't get the idea, try this:
Build a library, fill it with all human knowledge. You take it elsewhere when you need a book from it, but the book is only a simplified copy. You don't understand the real book, and you don't need to. Nobody takes the real books anywhere because why would you, when there's a whole library there?
Now that library goes rogue and the maintenance machinery starts killing everyone any-fucking-where near it. Where the fuck did they all come from, you swear to god there weren't this many, and there weren't because they're using the library's information to fight their war. The government fights a battle that destroys the planet against these robots and tears apart the library to stop them using it, only to be destroyed in the process. The library is leveled, cast into flames, every book burned and every computer virus-laden.
Then comes a man who worked there. He talks to the few surviving library workers, assembles their information, and starts rebuilding a city around the library and expanding it as the librarians find little scraps of paper and fragmented bits of files that stuck together just right read something. They rebuild a library from scrap on the ashes of the old. It isn't a shadow on the glory of the old, but it is all they have.
Then the city turns on itself, kills its master, and the librarians turn to rage. Half of them kill the other half and destroy the remnants of the library because where they're going they won't need science. Everything burns, and the city is left to a scattered few survivors, walls open to the world, with the hungry predators circling.
The Adeptus Mechanicus is the sole surviving librarian, desperately scrabbling through the ashes of paper and splinters of hard drives for anything to help him and the city he needs to survive just a second longer.
The Imperium isn't grim because things suck by choice and could be fine if a sensible person came along. That sensible person wouldn't survive fifty seconds of the reality. The Imperium is grim because every single shit decision, every single sacrifice, every single death, every single man woman and child suffering a shit life in the worst conditions imaginable, is the absolute best that can be done. It is a study of the worst happening to everyone and what part of your humanity must be sacrificed today just to stand a chance of survival, and all it asks is whether or not it would have perhaps been better to die."