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.)
This is an attempt to list all of the possible ways in which humanity may avoid scenarios where the average standard of living is close to subsistence, in response to Robin Hanson's recent series of posts on Overcoming Bias, where he argues that such an outcome is likely in the long run.
I'll start with six, some suggested by myself, and others collected from comments on Overcoming Bias and Robin's own posts. If anyone provides additional ideas, I'll add them to the list.
(I have a more general point here, BTW, which is that predicting the far future is very difficult. Before thinking that some outcome is inevitable or highly likely, it's a good idea to repeatedly ask oneself "This is all the ways that I can think of why it may fail to come true. Am I sure that all of them have low probability and that I'm not missing anything?" There may be some scenario with a non-negligible probability that your brain simply overlooked when you first asked it.)
Singleton
A world government or superpower imposes a population control policy over the whole world.
Strong Security
Strong defensive technologies and doctrines (such as Mutually Assured Destruction) allow nations, communities, and maybe tribes and families to unilaterally limit their populations within their own borders, while holding off hordes of would-be invaders and immigrants.
Non-Human Capital
Maximizing the wealth and power of a nation requires an optimal mix of human and non-human capital. Nations that fail to adopt population controls find their relative wealth and power fade over time as their mixes deviate from the optimum (i.e., they find themselves spending too much resources on raising humans, and not enough on building machines), and either move to correct this or are taken over by stronger powers. (I believe that historically this was the reason China adopted its one-child policy.)
Unlimited Growth
We don't completely understand the laws of physics, nor the nature of value. There turns out to be some way for economic growth to continue without limit. (Robin himself once wrote "I know of no law limiting economic value per atom" but apparently changed his mind later.)
Selfish Memes
Memes that manage to divert people's resources away from biological reproduction and towards memetic reproduction will have an advantage over memes that don't. On the other hand, genes that manage to block such memes will have an advantage over genes that don't. Memes manage to keep the upper hand in this struggle (or periodically regain the upper hand).
Disease, Warfare, Natural Disasters, Aliens, Keeper of the Simulation
One or more of these come along regularly to keep the human population in check and per capita incomes above subsistence.