IIRC, E-Man uses his last remaining strength in the final showdown to completely obliterate Horus, mind, body, and soul, so that Chaos could never bring him back.
Retconning it so that a repentant Horus becomes the new Big E would be worse than what J.J. Abrams did to Star Wars. Never mind that that would be completely out of character for Horus, who has built his whole rebellion upon his galaxy-sized ego and notion that Big E fucked him over.
Like I said (though feel free to dismiss my opinion because I'm still new to the hobby), Horus is a narcissist. If he mogged the Emperor, he would make it known.
I don't think the distress he had over the visions Chaos gave him were because the Imperium had become a fundamentalist loony bin, but because he was shown as being denied the glory he thought he deserved.
He echoes Jesus' words on the cross: "my Father, why have you abandoned me?" But for all the wrong reasons. Horus at that point stopped caring about the Great Crusade or the Imperial Truth.
Again, I don't claim to be a loremaster, and I know GW has retconned shit before, but completely departing from the most pivotal event of the entire IP just defies all logic to me.
(Which, in Current Year, means I think it will somehow happen).