This is a fundamental tenet of the market, which we might call the "free market" to emphasize that people should be allowed to participate without coersion or external meddling.

I understand the premise, but I would argue that such a thing can not exist, since manipulation for gains is a rational approach to any market. When the goal is to maximize profit, a free market system can be seen as an undesirable setup. For any given player, a market that they can influence to their advantage is more profitable than one void of meddling. That is, a 'free market' requires altruism, and the checks and balances intrinsic to a 'free market' can be overcome with collusion. As a result, regulations are proposed, and a healthy balance is typically the goal.

I suggest an eventual 100% marginal rate for reasons of political stability. That is, I think a healthy democracy requires that power not become too concentrated, and in many ways, money is political power. The Marginal Rate in the US has varied considerably, and has at times been over 90%. I'm not sure that there is evidence that it suppressed economic activity. Since the government spends that money, it might be a wash. I'm not really vested in the economic argument. I am more convinced by human behavior, and the consequences of a very disparate society.

Why worry about the health of this capitalist? He will be fine.

I'm not worried about the health of the extremely wealthy, but do worry about the centralization of power that comes with the concentration of capital. Regardless of its origins, Central Planning (the interests of few governing many) is usually a bad thing.