Because u can create as many theories as possible, but we can clearly see the winner now. Whatever The Emperor wanted is failed. Chaos, on the other hand, Have everything. If u see 2 villages > one of them starving to death, and other prospering, isnt it clearly obvious, who won?

Chaos gained everything and lost nothing in Horus Heresy, The Emperor, on the other hand, gained nothing and lost everything.

Chaos doesnt have I win button. It already won

yes they have a chance: because they are the "I WIN" button if they do that they win.
but that will NEVER happen in the background.
because war 40k is not supposed to have a winner.

chaos have an I win button: in fact have multiple i WIN button: but the fun part about chaos that chaos cannot really win because is the nature of chaos itself that make that impossibile.
chaos can win only if ONE god win. in the chaos there no "we win" there is only I win.

If someone win even a chaos god: chaos lose.

Because if someone win: it's no more chaos, it's order! the warp become calm and still, no more chaos.

so we can tell that chaos already lose. because chaos need the eternal conflict for survive, if the great game stop chaos stop to exist.

No one can win the great game: so chaos gods are losers because chaos gods are doomed to disappear, if one prevail all gods fall.

Dude this makes no sence a god made from the souls of a few(?!) million CEldar in his newborne state will manage to overpower a god who was born by the whole eldar civilization prefall? i mean really? unless ofc Ward is included i dont see the possibility of succes...actually Ahriman has better chances of becoming a god than the eldar freeing their souls...

this is how the Ymnead is described: all "i win" button are the final weapon of a faction from their point of view.
this is how war 40k work. All faction have their "i Win" insider their codex.

Then their point of view is F.U.B.A.R. and that is without taking into account that all CEldar need to die to make it happen...damn Slaanesh is going to have stomach problems after this...

But thats the point. Your described factions NEed Iwin button to have some hope. But for chaos, Eternal War is victory. They do not need to win. For as long there is a conflict > chaos is already won from it.

Great Game is eternal. No god would ever overcome others. This is nature of chaos. The state of the universe in 40k > is most beneficial for chaos gods.

which proves that if the emperors great crusade had succeeded, chaos would be destroyed. You just justified the emperors actions

Actually chaos was doing ok before the GC and doing far better now if anything the emperor just stirred the hornet's nest...

That is completly irrelevent to what i said. If the GC had succeeded chaos would be destroyed. So the emperor was right in starting the GC

Weakend maybe destroyed no.Because chaos is basically the reflection of sentient beings of the matterium in the warp ...so as long there are sentient beings with souls chaos will exist.