Let's back up: The way an endowment works is that donors donate money, which goes into a more-or-less permanent investment fund. The interest from the investment fund is then used to a) fund mission-aligned programs (in our case, OSS), b) stay ahead of inflation, and c) pay operating costs.
Where are you seeing capitalists "extract a slice of the pie" here?
"pay operating costs" is one place non-profits often find fraud. Getting the money into the market between donors and builders, now you have to pay professional investors. You don't get to 7-8% returns without equities, what happens if the market tanks?
Why not build something super minimal that requires less management and operating costs? That doesn't have the market risk at the center of it all? That doesn't have more points for fraud and abuse?
Can you explain the 2-3% gap between expected returns and outlays? Seems like a lot more than what is needed for accounting (based on the other main person here posting)