Actually, a "wealth cap" is not such a ridiculous idea. Essentially, it's simply a progressive income tax with a tax bracket at 100%. If the bracket is high enough, e.g. $200 million, then it doesn't act as a disincentive and helps to balance the imperfectness of the market.

Many would cite John Rawls here, but I have my own criticisms and my philosophical debates (I am a philosophy minor) can run into weeks of time. Instead, I'm going to simply make the argument that wealth over $200 million is meaningless to a single person. That is, from a practical perspective, that is more money than one could ever spend. The only positive use that could come from that amount of money is heavy watched investment and philanthropy. It is not easy to see that $1 billion is enough to move the world. Why should that responsibility fall into a single citizen's hands? In the best circumstances, the citizen hires a flotilla of people to manage that wealth (e.g., Gates and Buffet) and they spend it wisely and do great good. I don't see the fundamental reason why the government couldn't just do the exact same thing. By awarding the money of a government to a single person (Gates and Buffet each had, at the height of their wealth, more money than most of the governments in the world), this simply ensures an entrenched oligarchy of the rich (and therefore powerful) unless they decide to give it away.

Many would argue that this is "their" money so they deserve it. I think this argument is mostly bollocks, though. Bill Gates had over $100 billion at one point. Let's say that a skilled professional makes ~$1 million (gross) over the same time period. That's a 100,000x multiplier. Did Gates really do 100,000x more work? Did he really have 100,000x more insight? I think he just got lucky. He was at the right time at the right place with a good idea. From there he built a company. But the trick is that it's a lot easier to make a lot of money when you already have a whole lot of money.

Anyway, I'm gonna end this here, even though I'm not completely satisfied with my argument because this is becoming ridiculous.

P.S. This is not for those who believe that the government can do nothing right. I'm not sure that it's even worth arguing with those people. Theoretically, there's no satisfying proof that that's the case and practically the only evidence I see is that when those same people set the government up to fall, it falls hard.