This is a scheme to tax capital gains, not wealth. The point of this prescription isn't that "the rich have too much money," it is that we have gotten to the point where some of the largest capital gains go untaxed, period.

The idea that a gain isn't a gain until the asset is sold is nonsense. That'd literally be like saying that Bill Gates or Warren Buffett never made a dime on any of their stock that they are giving the charity. It's a ludicrous conception of capital gains, but it's exactly the one we use.