Whether or not a period of calm actually reduces their power, it does seem to have an effect on how much they in turn influence the materium. I put forth that while their power is constant, their reach is directly tied to the flow of emotion at any given time period. For example, Slaanesh had little to zero reach in the time before his birth since the eldar were the first to practice hedonism on a large enough scale to make his power truly felt in the Materium. We have no direct references to anything much earlier than that, but I would presume the other gods felt similar limitations during their own first reaches into our universe.
That's a pretty solid theory, and one I agree with.
My point being, if the source of their power is reduced (such as by an all consuming galactic predator or a race of genocidal alien machines) I believe such limitations could in time affect them again. Even if not completely, enough for someone else to gain the final advantage and take it will be all they need.
My question again would be: is that a loss? I don't think we can say that losing influence over the material realm is something that Chaos would be overly concerned with.
Warp Gods have died before.
Indeed they have. But the Chaos Gods have not. The Eldar pantheon is a different beast from Chaos, what holds true for one does not necessarily hold true for the other.
But that's irrelevant, as I said death is not required for a race to have lost.
It's not required but certainly death does equal a loss. I'm not denying that another faction could claim victory, possibly for a very long period of time but eventually they will all die.
And you're assuming that the other races WILL die when you cannot confirm this. We have ONE example of a c'tan dying under extrenuous circumstances, an occurrence that had never happened before or since. Because of that do you then take it as a guarantee that all of them will eventually die as well?
It's simple physics, everything decays given enough time. Sure that time could be billions upon billions of years, but it will still happen. Given a sufficient period of time any act, however improbable, can be considered guaranteed. The death of the C'tan is highly improbable so clearly it will require a very long period of time to be considered guaranteed, but it will still happen.
Well to be fair, the followers and daemons are who I was actually referring to and I said that a few times. You brought the gods into it.
Well to be fair, your OP just refers to Chaos and mentions both the Gods and their followers, which is at least a little ambiguous.