If you assume they can make 10% annually on that wealth (probably optimistic), that's $200B/year.
It is totally implausible given the history of U.S. tax policy and compliance, but let's say you could capture a 50% effective rate of that.
That's $100B/year: Not even half the increase in U.S. tax revenue between 2013 and 2014 due to economic growth[1]. Did that extra $200B enable Congress and the President to solve significant problems last year?
[1] And whether you believe in the Laffer curve or not, it's hard to imagine that levying a massive tax on billionaires' investment income would have a positive effect on economic growth & thus tax revenue from the non-super-rich.
There was recently an op-ed in the New York Times about this very subject.+ I encourage you to read the comments. Every time the issue is raised you get the same predictable whines about how tough life is on two or three hundred thousand dollars a year. A typical complaint is about how expensive Manhattan apartments are, which is like complaining that it is hard to make ends meet after making the monthly Ferrari payment.
+ http://www.nytimes.com/2015/12/28/opinion/campaign-stops/250...
But isn't there an argument to be made that when these ultra-rich are stripped of their billions, they are less likely to spend money trying to do things to bend laws to their benefit?
If Koch brothers / Soros did not have the billions they do, they'd spend a lot less gaming the system, thereby the larger population of non-rich having greater power in the system, in essence ensuring better democracy, do you agree?
I think we have exactly the government one would expect from taking account of the demographics of voters. 2/3 of voters in midterm federal elections are 45+ and the median voter in municipal elections is pushing 60: https://www.minnpost.com/eric-black-ink/2015/07/tracking-tur....
Given the demographics, I have a hard time seeing what exactly would be different if the rich had less power. I mean, are we going to raise taxes to make health care free for younger people, when the median voter is going to be eligible for Medicare in a few years anyway? And is it really rich people lobbying for more surveillance and resisting decriminalization of drugs, or is it my mother?
While your mother may be voting for such policies, she is likely not lobbying for them (or writing them). The companies and high net worth individuals who have a financial stake in these policies are the ones doing that.
Every piece of legislation that exists benefits a rich person in some way.
[1] Which they do by a huge margin, with the sole exception of marijuana: http://www.huffingtonpost.com/2014/04/17/drug-legalization-p....