Chaos is many things, my child. The Gods are many things. Among those things, they are all within us - spurring us on. And you are flawed in your perception of the Gods of Chaos.
Khorne was the first of the Gods to come forth. And, he came forth around a thousand years ago from our time, some time in Terra's Middle Ages. In other words, Khorne was born from emotion. The lust for battle, the honor among warriors, and the bloodthirst of the crazed.
As such, every God is woven together of their respective areas of emotion. Without the very things that give them substance, they would cease to exist. Why else would the Eldar hope to prevent humanity to expand? Why else would the Cabal try to convince the Alpha Legion to help Horus win over the Emperor, his shame then wiping out humanity - dealing a cripling blow to the Gods of Chaos.
Take away the thing that nurtures a being, and it will summarily perish.
You understand nothing of the Gods, peasant!
I won’t traverse the route of patronization, as amiably amusing as it may seem, because frankly the argument of the birth of the Chaos God`s during Terra`s early history is ridiculous, and there is not a single source to support such. Hypothetically, as I am sure I have spoken to Child-of-the-Emperor on such a matter before, the factors needed for the creation of Slaanesh cannot feasibly be placed upon the human race to account for the birth of the Lord of Skull`s, Decay and the Fate.
The Eldar Empire spanned the entire Galaxy, a miasma of countless worlds rivaling if not superior to that of the Imperium during the zenith of the Great Crusade; billions of these beings, highly attuned to the Warp and still it took millennia of debauchery and hedonism before they created Slaanesh, still the weakest of the Ruinous Powers.
And yet, upon this basis, I am to believe that the three most powerful Chaos Gods: Khorne, Tzeentch and Nurgle in descending order can be accounted for, for a sparse few centuries at best, upon Terra alone, by but a billion, short-lived and spiritually weak humans? It is absurd, and I believe holds not grasp upon current fluff
As such, the Ruinous Powers, Slaanesh exempt were born into a realm before time itself had been forged; there was no life, or anything sufficient enough of providing the essence of emotion to be reaped by the Gods.
The Chaos God`s will perish? They are the omnipotent masters of the Warp, the champions of manifested madness, the lords of that which cannot be bound by theory and reason you
cannot put mortal concepts upon them, for they are not so easily labeled.
Why did the Eldar not wish for the Imperium to expand? I have not read Legion, though frankly Farseer Eldrad Ulthran of Craftworld Ulthwe attempted to save Primarch Fulgrim from the predations of Slaanesh, for it would prevent the Galaxy being induced by bloodshed spanning tens of millennia (a reference to the never-ceasing slaughter of the 41st Millennium). Why did the Eldar not want the Great Crusade to assert dominance over the Galaxy? Maybe because most of the Exodite and Maiden Worlds were yet to be conquered? Maybe they feared some form of repercussions of the Emperor`s attempt of jacking into their matrix, the Webway, do we have any solid evidence?
In all honesty, I lack the sources to give a fully-encompassing and definitive answer, with little or no flaws, but by removing the nonsense that is the birth of the Chaos Gods during the youth of Terra, helps destroy the lower echelons of your theory.
That said, its pleasant to have a complex debate
