Somone said earlier that the Astropaths are 'gifted some of the Emperor's power' or something like that... that's the soul-binding, but all that does is give them some protection from possession (which doesn't always work), and yet there are plenty of psykers out there who haven't been soul-bound and yet get by perfectly well. Granted, they are the more powerful ones, but Astropaths must ave a reasonable level of power to transmit messages across large chunks of the galaxy, surely?
To add to my point earlier about the High Lords or whoever else surely having a back-up plan, how do we know that the Emperor and the soul-binding are actually essential? The Imperium is all about social control through secrecy and disinformation, and telling the populace, and their leaders, at large that all of Human society depends for survival on the Emperor, and that warp travel and interstellar communication will break down without him, would tend to suit their agenda to remain in power.
There was fluff ages ago in various places about the whole 'Star Child' thing, and various bits of the establishment being terrified of some of the more ambitious Thorian hopes to regenerate, reincarnate, or maybe even replace the Emperor, as a walking, talking Emperor (or replacement) would remove any need for High Lords, so some of them are liable to face a sharp reduction in status, wealth and power. Could this whole dependency on the Emperor simply be propaganda as part of the same agenda?
And, yeah, who the Hell was generating the Astronomican during the Great Crusade, and even just after the Emperor's death? They didn't build the Golden Throne overnight, some background has it that the Emperor maaged to retain consciousness logn enough to tell Dorn to build it, and yet Russ at al managed to navigate their way to the Eye of Terror in pursuit of the Traitor Legions. Even if the Emperor was generating the beacon all the time, he was evidently manging to do it without aving to rather vampirishly absorb the life-force of a thousand-odd psykers per day, and if his psychic presence remained pretty much as it was regardless of his physical state, then why the sudden need for that when interred in the Throne?
Sorry, but too many contradictions. It's all a conspiracy; someone call David Icke.