That doesn't really work with the descriptions of those empires though. The human stellar empire was greater and more powerful than the imperium at its height, and spread far further than the current astronomicon range over millions of worlds. Similarly, the Eldar at the height of their power were creating artificial worlds, terraforming maiden worlds across the galaxy, and generally acting as gods. There's no way the human empire would have been ignored and allowed to spread all over the galaxy like they did - the Eldar would have swatted them out of hand. You don't have two all powerful galactic empires comprising of millions of worlds going about their business not interacting at all. It would be weird beyond belief to think the Eldar just let humanity spread over the entire galaxy like they did.
(Timeline issue also comes up with the orks - they were supposedly created by the old ones as part of the war in heaven, yet in 60 million years since haven't engulfed the galaxy in green? Presumably the Eldar empire, and whatever other support powers arose and died in that 60 million year gap, kept them in check?)
I also wonder whether the current state of affairs with the warp is temporary - if we're still seeing the aftershock of the birth of slaanesh. For the vast majority of history the warp was a non-hostile place accessible and useable by sentient races despite the original three chaos gods being there. Maybe the current madness and power level is a temporary state of affairs and the warp will slowly coalesce back to its calm, safe state over time? (As presumably it did when the other chaos gods formed).