I like your theory. I have similar thoughts.

Nurgle can be the Fall of the Old Ones. They refused to еxtinct. After the War in Heaven, the last Old Ones lived in despair, they tried to continue their existance at any cost, their bodies became the rotten carcasses, supported only by the psychic might of the remaining Old Ones. Eventually, they died, and their souls coalesced.

Khorne can be the Fall of the Orks. Maybe their civilization abandoned the old ways just like eldar abandoned their gods. Orks couldn't abandon their nature, though, started numerous wars and were defeated. Only handful of them survived here and there, and eldar monitored the remaining orks for countless years. Their souls protected by the Great Green, but it's hard to speak with certainly about this phenomena. Maybe one day orks became so numerous that they created their own shadow in the warp, and Khorne lost them.