I can never understand why the idea that replicating systems might just never expand past small islands of clement circumstances (like, say, the surface of the Earth) gets so readily dismissed in these parts.

People in these parts don't necessarily have in mind the spread of biological replicators. Spreading almost any kind of computing machinery would be good enough to count, because it could host simulations of humans or other worthwhile intelligent life.

(Note that that question of whether simulated people are actually conscious is not that relevant to the question of whether this kind of expansion will happen. What's relevant is the question of whether the relevant decision makers would come to think they are conscious. For example, even if simulated people aren't actually conscious, after interacting with simulated people intergrated into society all their lives most non-simulated people would probably think they are conscious, and thus worth sending out to colonize space. And the simulated people themselves will definitely think they are conscious.)