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 know that the Chaos Gods were linked to the current turbulence in the warp, as their power is dependent on the flow of emotion. The Warp was described as calm during the Old Ones' reign, and for a time when Mankind was expanding his empire through the stars. The main support for this is that they have zero mention during the War in Heaven and that Mankind was able to travel the warp relatively safely enough to colonize the galaxy.
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.
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.
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?
Warp Gods have died before. But that's irrelevant, as I said death is not required for a race to have lost. 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?
I think it's a mistake to conflate the Gods and their followers in this case.
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.