One of the issues is that their value isn’t fixed. A billionaire as relatively little in the way of liquid (or liquidatable) assets. Their company might be worth billions, but, by taking it, you will destabilise it. Its value will plummet.

In order to access that money, you need to syphon it off more slowly. Think of the goose that lays golden eggs. Cutting it open won’t get you a glut of gold. The counterpoint is that you still need to collect the eggs!

In my opinion, we need a tax setup that forces individuals to regress to the mean. (Default is the rich and poor both move towards average when they are of average performance).

We also need to force companies to follow a power law. A few big companies, with the number growing as you move down. A tax setup that punishes forming big conglomerates, and so encourages more medium and small companies would be optimal. Have it adjust based on the overall industry. This both keeps industries competitive, and syphons money from those most able to bear it.

There is a huge difference between knowing what is needed, and how the fuck to implement it however!