And yet is is possible. The warp is an impossible realm where anything is possible. I'd wager a C'tan could be defeated or at least sufficiently weakened by some kind of warp based weapon.
The Talismans of Vaul couldn't defeat one c'tan when there were twelve aligned against it. Tech like that simply can't be built by anyone short of the necrons anymore, and even that is not confirmed.
Breaching a shard's necrodermis will 'scatter it to the galactic winds'. pg.40 of the Necron Codex. This may not mean they're destroyed though i believe it means they are but it certainly defeats them.
Granted, though I doubt this "kills" them I will concede that it for the most part removes them from the equation.
You mean millions of necrons. Page 7.
My mistake, I was thinking of the later entry.
If the necrons 'only' lost millions to shatter and imprison the C'tan i'd wager the servants of chaos who number in the billions or trillions, not to mention the infinite number of daemons could eventually do it, especially with actual, inviolable gods on their side.
Short of the Chaos Gods literally stepping into the Materium to strike the killing blow to each and every c'tan shard to permanently destroy them, I simply cannot see this happening. The Chaos Marines and Daemons would have to work to breach every last necrodermis body to take the c'tan shards out of the fight.
True, the C'tan free would cause horrific damage. Would they be able to meaningfully defeat the Ruinous Powers though? I don't believe there's any mention of the Great Work in the new codex. The 4 are actual gods, immortal and very possibly omnipotent and omniscient or near enough not to matter. By virtue of being undefeatable surely they win by default?
The Chaos Gods don't care enough though to make that kind of direct assault. Which means it's up to their servants and mortal followers, warriors and slaves who CAN be defeated. The Chaos Gods' power is only as strong as the flow of emotion allows.
The argument here seems to be that the gods themselves need to be killed for them to not count as contenders. This isn't the case. As you said, the c'tan can be breached and scattered, this doesn't necessarily mean they're dead but it does place them in a position where they cannot affect the scales to anywhere near the same degree as if they were still in necrodermis shell. Do you see?
By lasting victory, I mean holding the lion's share of the galaxy at large. For Chaos, this would mean their servants
have the biggest holdings, perpetuating war death and whatever else the gods want on a galactic scale. There is still a chance that the necrons could amass enough forces to drive the Chaos Legions to breaking point, or the tyranids could overrun all life and starve the gods and therefore their servants of enough power to continue the fight.
You're arguing the gods cannot be killed makes them automatic winners, but keep in mind that they held very little sway in the time before the War in Heaven and for a long time after. Such a time could easily arise again under the right circumstances.