And second: What if other systems are less corruptible and exploitable? How do you know whether all systems have the same potential for corruption and exploitation?
Capitalism is a system that explicitly rewards greed, and that gives the most power to the people with the most to gain from exploiting said power. Why is it unreasonable to believe that other systems are likely better on these fronts?
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.
Funnily enough, in my eyes it's obvious that capitalism in a globalized world must inevitably lead to unfettered capitalism, since the gravity effect of money along with competition between countries for their investments (with tax breaks etc. ) inevitably allows the capitalists to change the laws in their favor more and more. So I would ask for an objective measure of "how much in control humans are". I don't think it's a single dimension like you're implying, and I don't think capitalism is near any optimum due to the aforementioned effect.
Which non-capitalist system does not ultimately concentrate wealth and power even further? And, if nation-states are organisms, and survival of the fittest is a law, which of those systems can survive in the real world? Excepting perhaps those propped up by a disproportionate natural wealth and homogeneous culture, where it is easier for people to continue to believe in "our good" when hard times come.
Capitalism is the only system that pretends to respect human nature - gives us a lever that says, "want more stuff? provide more value." It gives the self-interested a healthy outlet that the other systems lack. It must be managed, though, by power, and when the two collude you get fascism. So, I guess maybe I share your view, that lobbying breaks the system.
IMO it's not possible to justify your claims without taking a whole tower of (IMO likely false) assumptions for granted. I'd love to go into detail on each assumption I see, but I've had similar comments regarding flaws of capitalist systems get derailed too many times by very angry commenters who insist that such thoughts are not worth discussing and MUST NOT be discussed.
The naive view is to think that lobbying is automatically bad. Policymakers can choose who they talk to. If you're worried about corrupt policymakers, you've correctly identified where power resides: in government.
All I did was disprove your theory that private companies have no power, and that you or I have the same options to influence the government.