That's fine to have your own opinion. Stating your opinion as if it was fact (particularly the use of "easily") is not.
The codex merely states that Altansar "rode out the initial psychic shockwaves" of the Fall and that it "was subsequently caught in the gravity well of the Eye of Terror".
You stated the source, Codex Eldar 4th edition. The Craftworld was so close to the EoT it was sucked back in. Thats as close as you can get. Period. Yet none of those Eldar instant died like some craftworlds stated to have caught to close.
Page 8 of the Codex states that "All Eldar within thousands of light years [from the epicenter] were reduced to lifeless husks...Even those who had foreseen the catastrophe and fled upon the craftwords were overwhelmed, with only those furthest from the devastation surviving."
So we know that 1. Altansar was at least several thousands of light years away from the center, and that2. it was still sucked in some time later.
That's it. Nothing about it being too close to the epicenter to be destroyed.
Do you have any clue how far several thousand Light Years is? Its not far at all my friend.
"The Milky Way contains over 200 billion stars, and enough dust and gas to make billions more. The solar system lies about 30,000 light-years from the galactic center, and about 20 light-years above the plane of the galaxy. More than half the stars found in the Milky Way are older than the 4.5 billion year old sun."
Seriously, 7 thousand Light Years is barely any distance overall. Also Terra which is close to the EoT was UNTOUCHED by the soul sucking as well. Even though it states all these Humans and Orks died with Eldar as well.
There we probably humans around where the Eye of Terror is (this being during Old Night) and we all know the Orks are everywhere. I wouldn't find it surprising billions of humans and Orks died when the Eye of Terror came to be.
Several reasons I could think of:
1. Slaanesh was born from "suckling upon the dark fodder of the Eldar spirit". There's a direct connection.
2. The "psychic shockwave [of Slaanesh's birth] focused on the Eldar"...
I mean, to use a bit of deductive reasoning you're so fond of, how could the Eldar go from a galaxy-dominating civilization to know they are "doomed" if some 25-30% of the civilization survived? That's a significant portion.
I explained above the deductive reasoning of Glaxy length to Eldar populating all the Galaxy in some form or another. As for why the Eldar decline, I explain already from the Codex itself. it has nothing to do with numbers, but it has to do with several facts.
1) Eldar low Birth Cycle is laughable to the other races. Meaning in time they would be overwhelmed anyway.
2) Eldar lost the major numbers with the fall, and were in disarray as a civilization. They also lost their Warp Gods. That stings.
3) The other races after the Fall had the time and space needed to boom out of control to overwhelm the depopulated and disarrayed Eldar.
This is all explained in the codex wording when read. At least to me.
Remember, their technology, culture, and other knowledge remained in tact after the Fall. It's not like humanity where we've forgotten how to manufacture most of our technology.
It's purely a number reason why the Eldar are a weakened race.
Yes, there is a number of reasons. I never said otherwise. Eldar after the fall were not incompetent though after 200 years (toward the end of the Great Crusade) which is the Original Topic is discussing. They could and should beat back the Emp easy after 200 years of foresight, Aspects being made, and relationships reforged. As well having more Eldar Numbers 200 years after the fall compared to 10,000 years after the fall.