That is a problem, which is why a wealth tax should probably only be applied to the very rich. And then, yes, the point is to force them to sell assets, thereby reducing their estates; a large inheritance tax would have a similar effect. A main point that Piketty makes is that even a tiny wealth tax would be extremely useful (e.g. .1%) because then the wealthy would be forced to reveal their hidden offshore assets. Have very large penalties for tax avoidance. Also, some would just renounce their American citizenship, so probably also need a large one-time penalty tax for that. It probably only works if much of the world agrees to do it. France had one, but the wealthy objected/left and then they stopped it. The main things is that people realize that it would be good for everyone if the wealthy had a smaller share of the wealth.