So in this scenario, a "capita" would be like a hive mind? Interesting, but what prevents such "individuals" from being driven to a subsistence standard of living by Malthusian dynamics operating at this higher level of organization?

This is not exactly an answer rather than a comment: Malthusian dynamics implies reproduction of approximately fixed-size (where size includes needs and abilities) individuals.

A hive mind differs from those premises in that one doesn't necessarily need to reproduce, just grow. It's not immediately obvious that a mind housed in the equivalent of 1 billion human bodies applies the same kind of arithmetic to resources, happiness, etc.

I don't have a theory to propose, as we don't really know that much about non-human minds, but, for instance: If I "added" to myself another (mindless) body, over which I had unusual control (e.g., I could choose whether or not I feel pain from it), and it was worse (say) fed than I am, I don't feel my "average well-being" would decrease. However, for two individuals it would.

As a closer-to-reality example, if I were to add some cells to my body that are closer to "subsistence" level (e.g., by eating the same but starting body-building), from a cell point of view the median & average "wealth" decreases, but from mine it may raise (if, e.g., chicks dig me more).

The point being that it appears that the resources-per-replicator may decrease yet resources-per-individual increase. Aggregating replicators in individuals at least complicates the math enough that I'm not convinced that's very improbable.

(Sorry for the long comment, but I got another point of view:)

Even though Malthusian dynamics work somewhat at the level of cells, I don't give a damn about my individual cells themselves. It just doesn't enter the calculation. A "hive-mind" aggregating all humans might be the same: a great discontinuity in how calculations are made. (Granted, the result is, in fact, a singleton.)