Even if you discard the notion of morality as human-only, and assume alien civilizations would not necessarily develop it too, there is still a game-theoretical

Even if you discard the notion of morality as human-only, and assume alien civilizations would not necessarily develop it too, there is still a game-theoretical reason for why the Dark Forest state is not inevitable. The Dark Forest state depends on the resources of the universe being scarce relative to their need/demand by all the sentient species in the universe. In the novels, survival of your species requires killing any other species that would compete with you for those resources. But given the extraordinary abundance of resources in the universe, it's not a given that demand would overwhelm supply so much as to necessitate such an extreme survival strategy. There is a massive amount of matter and energy in the universe. Further it's likely that super-advanced species with the technology to manipulate the Strong Force of matter, as even the relatively less advanced Tri-Solarans could in the novels, will have developed the ability to recycle and reuse resources far more effectively than we have today. The ability to build things using the Strong Force implies the ability to disassemble and reassemble them at the subatomic scale. That pretty much solves all known problems with recycling. Think Star Trek replicators on a massive scale. Now that resource starvation is no longer such a problem for species, participating in continuous mutual destruction of other species is far less optimal. It needlessly increases your own species' danger value to all other species, and the likelihood of being pre-emptively attacked and destroyed, for no good reason. Rather I suspect the optimal strategy is similar to what evolved on earth - develop effective technological deterrents, be they nukes, photoids, or dimensional collapse attacks or whatever, and then don't use them. Their highest value is in your enemies knowing you have them, but that you will not use them unless attacked first. Keep your own threat level relatively low, but without leaving you completely helpless and vulnerable.