You don't die if you fall in the Void do you? I can't remember off the top of my head what the Doctor had to say about the matter. But if you don't die as soon as you fall into the Void then you've got millions of very annoyed Cybermen and Daleks trying to figure a way back out and 'lo you end up with a rift between the universes and 3lots of Cybermen and Daleks wondering about.