If humanity goes extinct due to an existential catastrophe, it is possible that aliens will eventually colonize Earth and the surrounding regions of space that Earth-originating life otherwise would have. If aliens' values are sufficiently aligned with human values, the relative harm of an existential catastrophe may be significantly lessened if it allows for the possibility of such alien colonization.
I think the probability of such alien colonization varies substantially based on the type of existential catastrophe. An existential catastrophe due to rogue AI would make it unlikely that such alien colonization would happen since it would probably be in the AI's interest to keep the resources in Earth and the surrounding regions for itself. I suspect that an existential catastrophe due to a biotechnology or nanotechnology disaster, however, would leave alien colonization relatively probable.
I think there's a decent chance that alien values would be at least somewhat aligned with humans'. Human values, for example fun and learning, exist since they were evolutionarily beneficial. This weakly suggests that aliens would also have them due to similar evolutionary advantages.
My above reasoning suggests that we should devote more effort into averting existential risks that make such colonization less likely, for example risks from rogue AI, than from other risks.
Is my reasoning correct? Has what I'm saying already been though of? If not, would be be worthwhile to inform people working on existential risk strategy, e.g. Nick Bostrom, about this?
Regarding the likelihood (not the value) of intergalactic alien civilizations you might be intrested in this post on Quantifying anthropic effects on the Fermi paradox by Lukas Finnveden. E.g., he concludes:
The quote continues:
I think the 70%/30% numbers are the relevant ones for comparing human colonization vs. extinction vs. misaligned AGI colonization. (Since 5% cuts the importance of everything equally.)
...assuming defensive dominance in space, where you get to keep space that you acquire first. I don't know what happens without that.
This would suggest that if we're indifferent between space being totally uncolonized and being colonized by a certain misaligned AGI and if we're indifferent between aliens and humans colonizing space: then preventing that AGI is ~3x as good as preventing extinction.
If we value aliens less than humans, it's less. If we value the AGI positively, it's also less. If we value the AGI negatively, it'd be more.