I think your over-exaggerating chaos' ability to do anything on this scale. I mean the level of warp activity in the age of strife was only caused by the forming of Slaanesh. Besides that they would need to expend much energy to do anything of any scale. And only inside warp storms can daemons roam free. A galaxy wide one would do more damage than good.
And the Emperors death won't bring about a warp storm. Slaaneshs birth only caused one due to it being a violent god. The Star Child is a god which doesn't prey on humanity.
Agreed.