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while one can postulate that this level of civilization doesn't need planets as habitats, planets with biomass would likely be an important asset as they ease the supply logistics and act as a trampoline for further, faster expansion.

But once your civilization isn’t threatened with complete and sudden obliteration by planet killers, you’re no longer stuck in the prisoner’s dilemma and you’re not necessarily compelled to strike first. It’s a big difference.

You shouldn't assume that an intelligent civilization wouldn't find a weapon of mass destruction that's effective against a non-planet based civilization.

So the utility cost of the betrayal scenario goes from negative infinity to a question mark.

Outer solar systems bodies have water and other volatiles as major constituents. If you want raw material for human bodies, cattle, trees, plastics, that is where most of it is.

And so would interstellar bodies. I’m willing to bet that a large fraction of the mass of a developing solar system gets ejected during the early years of its formation. If you can freely roam among the stars you would probably find everything you need in interstellar space.

miller experiment shows that there's some convergence on certain molecules, it's safe to assume that while every specific biology will differ, the building blocks will look not that far apart, and transforming existing biomass into usable biomass is going to be more energy efficient than transforming raws into usable molecules.

You just described the cold war :) I don't have any larger point, but found it interesting.

What does a war between two nuclear-capable nations look like? If they can nuclear bombs, doesn’t that mean their first strike weapons are all nation killers? If we assume they still primarily use cities as the sites of their industrial and population centres then doesn’t that mean any city within striking distance are immediately obliterated by large, dark, unstoppable projectiles? Any point defence you can think of fails when you can easily throw thousands of these at a nation. There’s no way to win that game for either side.

Perhaps the winning move, like with today's nuclear weaponry, simply not to play.

Or to hide in the Dark Forest instead of announcing yourself to the cosmos.

This does seem to be the logical conclusion of a game theory analysis. Since planet killers can show up without any warning, the only solution is make it impossible to aim your unstoppable weapons, which means you have to be impossible to detect or locate.

So as you said, maybe the solution to the Fermi Paradox is that every surviving civilization is hiding in cloaked habitats within the vastness of intergalactic space.

By the time you can launch a planet-killer, you are no longer tied to your own planet anymore. It requires marshaling an amount of energy far beyond what you could afford to do in a biosphere, and a biosphere is all a planet really has to offer. Two of these civilizations might certainly take a swing at each other's planets, but it wouldn't end the war by any means; at this point the planet may not even account for single-digit percentages of what is going on in the system.

I think if you look ahead at feasible technology, you get self-sustaining pure-space civilizations long before you get civilizations that can launch relativistic planet killers.

(The other problem with the "planet killer" hypothesis, as we've discussed in other HN conversations, is that there's no reason to wait until there's a civilization on the other end to launch one, given how fast they can pop up. As soon as you see life, you should launch one. But Earth has been broadcasting fairly conclusive proof that a civilization with just a bit more tech than we have now is alive ever since the Great Oxygen Catastrophe, yet nobody has killed Earth.)

When some civilization becomes apparent, wouldn’t game theorizing say there’s a free rider problem? Wait for someone else to poke their heads out to handle it, save us the trouble, and maybe learn how we rank in a battle royale?

With nuclear weapons we had MAD - and even if the other side didn’t retaliate, the planet would be ruined by a nuclear war so no one could win.

This is more dangerous. If weapons are absolutely but selectively destructive - I can wipe your planet out quickly before you have time to respond and my planet is just fine - it incentivizes me to attack first and hunt down other potential threats and destroy them before you or they destroy me.

Yes but it also changes your choice of habitat lest someone do the same to you. Remember with clever aiming these weapons have limitless range so there is no such thing as a border conflict and someone could nuke all of your systems. My point is that this strongly discourages solar system based empires and favours mobile space based civilizations. My hunch is that our galaxy is filled with nomadic hard to detect habitats and that that is the natural progression. There’s probably only a brief period of time where technological civilizations remain bound to their original planet and solar system. That’s my answer to the Fermi paradox.

This is basically what I’m saying. With an alternative to unmovable planets, the Dark Forest scenario becomes a bit less likely because the betrayal scenario in the prisoner’s dilemma becomes a lot less extreme.

Why do you say "had"... MAD is still around. There might be less fear about it in the media, but the danger is still present. As defense technology matures, it might make nuclear war winnable for the side that controls a sufficient nuclear defense. Or it might not.