You realize what would happen right? The rich would divest from all their investments permanently. They'd sell all their assets foreigners[1] who aren't subject to the tax. There is no point risking losing your investment if taxes will take any gain. It fucks the expected value of investment.

Would you take a bet on a coin flip, if when you lost you paid a dollar, but when you won you'd get a nickle.

Banks would implode. The credit market would stop. Any business survive on a line of credit would just fail immediately.

Best case scenario, you'd just replace American rich for foreign rich.

[1] if you tried to tax foreign capital gains on American assets, you'd just destroy the entire economy. There would be huge lack of capital.

Your plan is insanity.

Edit: And some country would be creative and end tax treaties with the US just to get a couple trillion of capital flowing through their economy.