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.
Colonies and expanding populations likely become irrelevant once you approach the level of technology necessary for intergalactic travel. Quite possibly communication as well.