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.
1) The last time someone threatened Chaos, what did they do? Steal his sons, destroy everything he built, effectively kill him and remake his Empire in their image. If Chaos feels threatened, they will intervene.
2) As you say, the Gods don't care much about the material realm. Thus, can any defeat there be considered as 'loss' for them? If it isn't important to them, then it isn't a loss to loose it.
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?
But can the Gods be compromised to a similar degree? A C'tan without its necrodermis is almost completely powerless, a God without lots of emotion is simply hungry. Is a hungry foe a defeated one? A starving animal is at its most dangerous.
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.
I think it's a mistake to conflate the Gods and their followers in this case. The followers of Chaos are obviously fairly concerned with the material realm, and their fate is directly tied to it. I would call a victory for the mortal forces of Chaos unlikely. However, the Gods themselves don't need their followers to win. It doesn't matter one lick to them who is perpetuating war, etc. Khorne cares not from whence the blood flows and all that. It's the same mistake that leads to people calling the Horus Heresy a loss for Chaos, when it clearly wasn't.
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.
But is that a loss? The Chaos Gods have repeatedly shown themselves to be more interested in their own War than in the fate of the material realm. Does limiting their influence on the material realm (and for the record we don't really know how limited their influence was in the periods you mention) actually reduce their power? We don't know. Arguably it can't, since they are independent of time. Besides which: Everything else in the race can die. Given sufficient time everything that can die will. Therefore given sufficient time the Chaos Gods will be the only player left standing. If everyone else is dead, haven't they lost? If everyone else has lost, haven't you won?