Assuming agent costs keep declining exponentially as they have over the last three years, why would everyone not own some agents? It's not like the number of agents is capped and the billionaires hoard all of them. I imagine it would be more like smartphones, where there were only 50 million smartphones after the iPhone was first released in 2007 and now there are something like three billion. They become more accessible and plentiful over time. Same thing should apply to agents.

And in this world of abundant agents, what advantage does the billionaire have exactly over the non-billionaire? Their employees are less motivated than owner-operators, and they no longer have the scale advantage that large corporations used to have. Each individual can effectively operate like a large corporation, because each individual can have their own large synthetic workforce at very low cost. The scarce resource here then becomes uniquely human insights and real motivation, which entrepreneurs are always going to have more of than employees.

It's basic economics; larger firms become progressively less efficient for the same reason that communist command economies are inefficient, because there's no internal price signals to guide resource allocation. So there's a natural cap on how big a firm can get (in information theoretic terms, there's a hard limit on the amount of information a centralized structure can process effectively).