Quick thought: in some population models, in particular ones where human population peaks in the next 50-100 years and then continues declining (see Three mistakes in the moral mathematics of existential risk), there is a longtermist case to make that without life extension, the economy will not be sustainable when:
Elderly people comprise a large part of society and thus require more societal help
There are fewer people to fill niche sub-specialties
of course, a lot of things change with AGI as well, so it all depends on your timelines. I generally believe that EA doesn’t emphasize aging enough and would like to help you make the case if you’re interested.
A larger population has network effects: scientific progress is faster because a discovery by one person affects the entire population, economies of scale allow for better standards of living. The market is more efficient, which makes society as a whole more resilient to adverse events (e.g. during covid masks and vaccines were produced en masse very rapidly)