How do I know that a system maintained by people is exploitable by people? IDK, that's a real thinker, isn't it. That all systems are exploitable, in absence of human influence, isn't the argument I'm making. It's tautologically true that systems that humans design and implement and depend on the behavior of humans to continue to run would be exploitable by humans. You cannot design a system dependent on human action that is also immune to human nature.
As for the second, the more in control of the system humans are, the more they can exploit it. Your return on exploitation, which I think is really what you're getting at, is based on how diffuse the control and power is at the point that you exploit it. But the more diffuse control and power, the less responsive and effective the system is when working at full tilt. It's two sides of the same coin.
Capitalism is a system of decentralized resource allocation. Decentralized systems have their pros and cons, and other systems that would rely on centrally planned economies have pros and cons. Capitalism isn't the issue, unfettered capitalism is the issue.