The emperor was born when the human psyckers reincarnated all their souls into a single mega-powerful being. The emperor made grandiose plans to access the eldar webway, but was forced to abandon the effort when he was gravely wounded. Why doesnt he just allow himself to die, thereby reincarnating another emperor to serve and advance mankind. Perhaps the new emperor would not retain the old ones memories, but history can repeat itself. If humanity survived the end of the Dark age of Technology, surely they can survive a few thousand years, while the emperor rebuilds civilization. I could be totally wrong, any explanation would be appreciated.