Well, if that's you're main concern then allow me to instantly relieve you. The IRS does not accept stock certificates in lieu of cash. Section 14 of the Federal Reserve Act prohibits the US Govt. from owning any stock.
From the 3rd post in this thread. Also,
Annually, the stock market rises about 10% annually. An 8% wealth tax causes the stock market to only rise by 2% a year.
I'm not informed in the slightest on economics, but intuitively, there are several fundamental flaws in your logic in that
1. Billionaires do not own the entirety of the stock market
2. Wealth taxes are invariably progressive taxes
3. You can't directly add or subtract percentages of different "things".