I find all of these propositions questionable. It’s not clear at all that they will need to (1) reproduce (2) relocate or (3) capture an absurd amount of free energy. We can speculate they might want to do any of those but the arguments that they won’t seem just as strong.
I highly doubt there will be any disagreement about the merits and needs of colonization within a civilization capable of intergalactic travel—it will either be a good idea and they will agree or it will be a bad idea and they will agree not to.
Seeing no evidence of colonization (and knowing that if they all do it they If they will come into conflict with each other and risk their extinction) let’s suggest they all decide not to do it is a reasonable possibility.
Then timtyler’s point is easy to see: this isn’t so much about doomsday as about a change in society that devalues reproduction and expansion. There may be very few to no more humans born after say 2300 AD. And that’s because people don’t need offspring to work in the fields anymore, don’t fulfill their sexual needs like other animals, have incredibly inflated lifespans, etc.
“Children have gone from being productive capital goods to consumption goods. I don’t see any evidence that children are losing or will lose their value as consumption goods.”
Wait—the value of children recently changed fundamentally but we should expect no more change far in the future?
“I’m saying that the zero population growth faction will be a tiny minority by the time a civilization grows large and advanced.”
This does not reconcile at all with current population trends of developed nations. The UN medium projection for 2050 has the entire world at 2.02. Go ahead and assume people still want children as consumption goods, data suggests that not enough of them want this to maintain even zero population growth beyond the current century.