Originally Posted by Brother Lucian
My speculation is that theres just raw immaterium on the other side of the closed imperial webway gate. And even if shut down, it represents a place where the barrier between worlds is -quite- thin, and thusly a weakness should the Emperor cease pressing on it from the other side.
I can see that. I would think they could have used Geller field tech to seal it. Maybe just long enogh for him to I don't know get a sandwich, go to the bathroom, kill Horus. Maybe there are caveats to the tech that prevent it? I don't know much about Geller field fluff so.