On the Old World "predestination" thing, I think it's kinda bull. Yes, GW have gone out of their way to make many many parallels between WHF and WH40K. But the obvious parallel to Archaon in 40K is Abaddon. And Abaddon hasn't broken the Imperium and ended the setting.
Or hasn't yet. It might be the ultimate plan for 40K, but I hope not. It sucks.
If WHF had done gangbusters financially, I find it hard to believe that the End Times would have happened as it did. I think it's true that "Rocks Fall Everyone Dies" was one of the ideas they had going into the 90s. Just not the one true predestined outcome.
But maybe that is, as the 4chan /tg/ dwellers like to call it, "copium" on my part.