Even in a non-zero sum model, each investment needs to justify itself vs. alternatives at the time. I may have a wealth of investments that can have guaranteed positive returns, but I must choose which to invest in at any time with limited resources.

Even if I can expand available resources because of the promise of these alternatives, or through reinvestment of gains, I still must choose what to do at any time, until I have enough resources to pursue all promising avenues simultaneously.

Why the downvotes? Expanding Earth's economic sphere to all of cislunar space necessarily expands the resources and energy available to human industry. There will be returns, and the economy will expand. It's not zero-sum.