It’s bad because the only way this could work is by taxing unrealized capital gains on all investments, which will hurt non-rich people that invest.
I wouldn’t trust any government to not fuck with things retirement investment vehicles or property value increases if they started down this path, or to not put in a thousand loopholes that make it so the bottom 99% are the only ones paying it.
Remember, billionaires don’t have their money sitting in a bank. The vast majority of it is in non-liquid assets.