Time doesn't exist in the warp. By definition the Chaos Gods have always existed and always will. They are the warp and the warp is them. The Eldar Gods are different in that they were created intentionally (or at the very least, they seem to have been) and might have corporeal forms but even they they aren't necessarily dead they've just been consumed by Slaanesh (the fatality of that basically depending on your views of the Eldar gods).
I'm having trouble with the whole "Time doesn't exist in the warp" argument, in general and not just as you're using it in this discussion, as it's established as canon that the first Chaos God to emerge was Khorne, which didn't happen until sometime in early recorded human history. That means no Chaos Gods interfered during the entire War in Heaven, after the Necron's wiped most sentient life from the galaxy, or even during most of the Eldar Empire... although with the most recent fluff it's no longer clear to me of the order of those three events. I'm sure some other nasty form of the Primordial Anniahlator was at play considering it's reputation for billions of years prior to Khorne. Perhaps it's the creation of the Eye that gave Chaos such a preeminent role in the Milky Way where before they struggled to find material sponsors?
On another note, I wonder if the Tyranids can reemerge on a planet once they've cleansed it of all life? I think the Nids could easily win in the latest round by wiping life from the Milky Way, but sentient life has a way of hiding in the nooks and crannies of asteroids, Craftworlds, starship hives, etc. In that case, I think some current species would emerge from the Nid cleansing and repopulate the galaxy, although it wouldn't be one of the current players as they're known.
If the Nid's biological residue rises again after a planet they cleansed was repopulated millennia after they first ate it out of house and home. In that case, then the Nids will own the Milky Way in the Long Game IMO.
The White Zone is for loading and unloading only. If you have to load, or if you have to unload, go to the White Zone. You'll love it... it's a way of life.
: Cigarettes in space?
: It's the final frontier, Nick.
: But wouldn't they blow up in an all oxygen environment?
: Probably. But it's an easy fix. One line of dialogue. 'Thank God we invented the... you know, whatever device.'