I was talking about withholding, where I pay taxes that will never come due. On reflection maybe isn't a perfect analogy.

You haven't made a persuasive argument (or any argument really) against, you just keep insisting it's a bad idea.

One thing that stands out is you keep referring to "money that may never exist". That's not how tax works. You are taxed on the basis of your income, which is often but not always monetary. This is both intuitive and consistent with existing tax code. If you don't like it you have a much bigger problem than objecting to taxing unrealized cap gains.