[icanhackit]

> I doubt the idea of interstellar trade [...] Beyond a certain point of post-scarcity, everything stops being scarce, including invention and innovation.

Right but wouldn't you need interstellar trade to reach this milestone of post-scarcity? Otherwise you'd have to assume all forms of invention are achievable with the resources you start with.

Wouldn't game theory make you want to limit the resources and capabilities of your neighbor in case they try to do the same thing to you? So this idea of post-scarcity is never met because the uncertainties of attack from any number of other space-faring civilizations requires that you invest a significant chunk of your resources into your military. You could also have internal turmoil - the human race splits into distinct civilizations with different political, social and technological goals.