Well, I'm trusting Erebus from the 2nd Horus Heresy book and what he said on Davin to comatose Horus. Call me a heretic.
Imo Erebus was exactly right - The Emperor pretending that he didn't want to be portrayed as a god, but secretly wanting exactly that to happen. Erebus showed Horus exactly how the future was going to be.
So... yea, what the Emperor said and what he really wanted to me is a pair of two different shoes.
If I'm wrong, whats all that Star-Child-theory about? If the Emperor doesn't want to be worshipped as a god and doesn't want to become a god, then the Star-Child theory is a big joke, because the Emperor could decide to ascend to a Warp-God or not. If he wouldn't want, the whole theory is missing every base of discussion. You can't force someone to become a Warp-God, can you?
Anyway, it was more of a joke than anything seriously related to the initial question about soul-hunger. The Emperor was already multiple 10.000 years on earth before he started the psycher-killing. If it would be his age that requires him to do that, then he probably should have done it in his 30.000 years of living before already, not just in the last 10.000. The fact that it requires more and more offerings for me is just a sign that the Emperor (or whatever requires these souls) is losing strength (or getting greedy).
@Cthell: We don't know, but I think it's possible that Eldrad really is that old.
About the drain: Harlequin-souls are not known to wither within the Webway. We also know DE suffer from it harder if they are not within the Webway. So if anything, then the Webway works protective against the drain. Also since Exodiths and CWE do not have that drain, what else than the debauchery connection between the Dark Kin and Slaanesh shall cause it? It's not as if each Dark Eldar is far older than his Craftworld cousins either, so pure age can't be the reason.