It's already established that cogitators can compute short jumps of less than 5 light years. Navigators can probably extend this to some degree. But the key to the Astronomicon is that it allows extremely long jumps. Think about it: If it takes a lot of energy (fuel) to get into the warp, and then get out, then doesn't it make sense that the fewer jumps you make, the better your endurance? The largest battleships might eat so much power that making 5 ly jumps over and over might burn them out in short order. The old human civilization was made up of a confederacy, implying that they were still effectively isolated. They could communicate fine, and travel fine, but it took so long, and used so much fuel, that it wasn't economical. The astronomicon enables a centralized government more than it does anything else. By this logic the thing could shut down and the Imperium would disintegrate even as parts of it persevered. Only the threat of some massive, coordinated attack threatens human extinction at this rate.
